# 30 SWIG and Ruby

- Preliminaries
  - Running SWIG
  - Getting the right header files
  - Compiling a dynamic module
  - Using your module
  - Static linking
  - Compilation of C++ extensions
- Building Ruby Extensions under Windows 95/NT
  - Running SWIG from Developer Studio
- The Ruby-to-C/C++ Mapping
  - Modules
  - Functions
  - Variable Linking
  - Constants
  - Pointers
  - Structures
  - C++ classes
  - C++ Inheritance
  - C++ Overloaded Functions
  - C++ Operators
  - C++ namespaces
  - C++ templates
  - C++ Smart Pointers
  - Cross-Language Polymorphism
    - Exception Unrolling
- Naming
  - Defining Aliases
  - Predicate Methods
  - Bang Methods
  - Getters and Setters
- Input and output parameters
- Exception handling
  - Using the %exception directive
  - Raising exceptions
  - Exception classes
- Typemaps
  - What is a typemap?
  - Ruby typemaps
  - Typemap variables
  - Useful Functions
    - C Datatypes to Ruby Objects
    - Ruby Objects to C Datatypes
    - Macros for VALUE
    - Exceptions
    - Iterators
  - Typemap Examples
  - Converting a Ruby array to a char **
  - Collecting arguments in a hash
  - Pointer handling
    - Ruby Datatype Wrapping
  - Example: STL Vector to Ruby Array
- Advanced Topics
  - Operator overloading
  - Creating Multi-Module Packages
  - Specifying Mixin Modules
- Memory Management
  - Mark and Sweep Garbage Collector
  - Object Ownership
  - Object Tracking
  - Mark Functions
  - Free Functions

This chapter describes SWIG's support of Ruby.

## 30.1 Preliminaries

SWIG 1.3 is known to work with Ruby versions 1.6 and later. Given the choice, you should use the latest stable version of Ruby. You should also determine if your system supports shared libraries and dynamic loading. SWIG will work with or without dynamic loading, but the compilation process will vary.

This chapter covers most SWIG features, but in less depth than is found in earlier chapters. At the very least, make sure you also read the "SWIG Basics" chapter. It is also assumed that the reader has a basic understanding of Ruby.

### 30.1.1 Running SWIG

To build a Ruby module, run SWIG using the `-ruby` option:

    $ swig -ruby example.i

If building a C++ extension, add the `-c++` option:

    $ swig -c++ -ruby example.i

This creates a file `example_wrap.c` (`example_wrap.cxx` if compiling a C++ extension) that contains all of the code needed to build a Ruby extension module. To finish building the module, you need to compile this file and link it with the rest of your program.

### 30.1.2 Getting the right header files

In order to compile the wrapper code, the compiler needs the `ruby.h` header file. This file is usually contained in a directory such as

    /usr/local/lib/ruby/1.6/i686-linux/ruby.h

The exact location may vary on your machine, but the above location is typical. If you are not entirely sure where Ruby is installed, you can run Ruby to find out. For example:

    $ ruby -e 'puts $:.join("\n")'
    /usr/local/lib/ruby/site_ruby/1.6 /usr/local/lib/ruby/site_ruby/1.6/i686-linux
    /usr/local/lib/ruby/site_ruby /usr/local/lib/ruby/1.6 /usr/local/lib/ruby/1.6/i686-linux .

### 30.1.3 Compiling a dynamic module

Ruby extension modules are typically compiled into shared libraries that the interpreter loads dynamically at runtime. Since the exact commands for doing this vary from platform to platform, your best bet is to follow the steps described in the `README.EXT` file from the Ruby distribution:

1. Create a file called `extconf.rb` that looks like the following:

       require 'mkmf'
       create_makefile('example')

2. Type the following to build the extension:

       $ ruby extconf.rb
       $ make
       $ make install

Of course, there is the problem that mkmf does not work correctly on all platforms, e.g, HPUX. If you need to add your own make rules to the file that `extconf.rb` produces, you can add this:

    open("Makefile", "a") { |mf|
     puts <<EOM
     # Your make rules go here
     EOM
    }

to the end of the `extconf.rb` file. If for some reason you don't want to use the standard approach, you'll need to determine the correct compiler and linker flags for your build platform. For example, a typical sequence of commands for the Linux operating system would look something like this:

    $ swig -ruby example.i
    $ gcc -c example.c
    $ gcc -c example_wrap.c -I/usr/local/lib/ruby/1.6/i686-linux
    $ gcc -shared example.o example_wrap.o -o example.so

For other platforms it may be necessary to compile with the `-fPIC` option to generate position-independent code. If in doubt, consult the manual pages for your compiler and linker to determine the correct set of options. You might also check the SWIG Wiki for additional information.

### 30.1.4 Using your module

Ruby module names must be capitalized, but the convention for Ruby feature names is to use lowercase names. So, for example, the Etc extension module is imported by requiring the etc feature:

    # The feature name begins with a lowercase letter...
    require 'etc'

    # ... but the module name begins with an uppercase letter
    puts "Your login name: #{Etc.getlogin}"

To stay consistent with this practice, you should always specify a lowercase module name with SWIG's `%module` directive. SWIG will automatically correct the resulting Ruby module name for your extension. So for example, a SWIG interface file that begins with:

    %module example

will result in an extension module using the feature name "example" and Ruby module name "Example".

### 30.1.5 Static linking

An alternative approach to dynamic linking is to rebuild the Ruby interpreter with your extension module added to it. In the past, this approach was sometimes necessary due to limitations in dynamic loading support on certain machines. However, the situation has improved greatly over the last few years and you should not consider this approach unless there is really no other option.

The usual procedure for adding a new module to Ruby involves finding the Ruby source, adding an entry to the `ext/Setup` file, adding your directory to the list of extensions in the file, and finally rebuilding Ruby.

### 30.1.6 Compilation of C++ extensions

On most machines, C++ extension modules should be linked using the C++ compiler. For example:

    $ swig -c++ -ruby example.i
    $ g++ -c example.cxx
    $ g++ -c example_wrap.cxx -I/usr/local/lib/ruby/1.6/i686-linux
    $ g++ -shared example.o example_wrap.o -o example.so

If you've written an `extconf.rb` script to automatically generate a `Makefile` for your C++ extension module, keep in mind that (as of this writing) Ruby still uses `gcc` and not `g++` as its linker. As a result, the required C++ runtime library support will not be automatically linked into your extension module and it may fail to load on some platforms. A workaround for this problem is use the `mkmf` module's `append_library()` method to add one of the C++ runtime libraries to the list of libraries linked into your extension, e.g.

    require 'mkmf'
    $libs = append_library($libs, "supc++")
    create_makefile('example')

## 30.2 Building Ruby Extensions under Windows 95/NT

Building a SWIG extension to Ruby under Windows 95/NT is roughly similar to the process used with Unix. Normally, you will want to produce a DLL that can be loaded into the Ruby interpreter. For all recent versions of Ruby, the procedure described above (i.e. using an `extconf.rb` script) will work with Windows as well; you should be able to build your code into a DLL by typing:

    C:\swigtest> ruby extconf.rb
    C:\swigtest> nmake
    C:\swigtest> nmake install

The remainder of this section covers the process of compiling SWIG-generated Ruby extensions with Microsoft Visual C++ 6 (i.e. within the Developer Studio IDE, instead of using the command line tools). In order to build extensions, you may need to download the source distribution to the Ruby package, as you will need the Ruby header files.

### 30.2.1 Running SWIG from Developer Studio

If you are developing your application within Microsoft developer studio, SWIG can be invoked as a custom build option. The process roughly follows these steps :

- Open up a new workspace and use the AppWizard to select a DLL project.
- Add both the SWIG interface file (the .i file), any supporting C files, and the name of the wrapper file that will be created by SWIG (i.e. `example_wrap.c`). Note : If using C++, choose a different suffix for the wrapper file such as `example_wrap.cxx`. Don't worry if the wrapper file doesn't exist yet--Developer Studio will keep a reference to it around.
- Select the SWIG interface file and go to the settings menu. Under settings, select the "Custom Build" option.
- Enter "SWIG" in the description field.
- Enter "`swig -ruby -o $(ProjDir)\$(InputName)_wrap.c $(InputPath)`" in the "Build command(s) field". You may have to include the path to swig.exe.
- Enter "`$(ProjDir)\$(InputName)_wrap.c`" in the "Output files(s) field".
- Next, select the settings for the entire project and go to the C/C++ tab and select the Preprocessor category. Add NT=1 to the Preprocessor definitions. This must be set else you will get compilation errors. Also add IMPORT to the preprocessor definitions, else you may get runtime errors. Also add the include directories for your Ruby installation under "Additional include directories".
- Next, select the settings for the entire project and go to the Link tab and select the General category. Set the name of the output file to match the name of your Ruby module (i.e.. example.dll). Next add the Ruby library file to your link libraries under Object/Library modules. For example "mswin32-ruby16.lib. You also need to add the path to the library under the Input tab - Additional library path.
- Build your project.

Now, assuming all went well, SWIG will be automatically invoked when you build your project. Any changes made to the interface file will result in SWIG being automatically invoked to produce a new version of the wrapper file. To run your new Ruby extension, simply run Ruby and use the `require` command as normal. For example if you have this ruby file run.rb:

    # file: run.rb
    require 'Example'

    # Call a c function
    print "Foo = ", Example.Foo, "\n"

Ensure the dll just built is in your path or current directory, then run the Ruby script from the DOS/Command prompt:

    C:\swigtest> ruby run.rb
    Foo = 3.0

## 30.3 The Ruby-to-C/C++ Mapping

This section describes the basics of how SWIG maps C or C++ declarations in your SWIG interface files to Ruby constructs.

### 30.3.1 Modules

The SWIG `%module` directive specifies the name of the Ruby module. If you specify:

    %module example

then everything is wrapped into a Ruby module named `Example` that is nested directly under the global module. You can specify a more deeply nested module by specifying the fully-qualified module name in quotes, e.g.

    %module "foo::bar::spam"

An alternate method of specifying a nested module name is to use the -prefix option on the SWIG command line. The prefix that you specify with this option will be prepended to the module name specified with the %module directive in your SWIG interface file. So for example, this declaration at the top of your SWIG interface file:

    %module "foo::bar::spam"

will result in a nested module name of Foo::Bar::Spam, but you can achieve the same effect by specifying:

    %module spam

and then running SWIG with the -prefix command line option:

    $ swig -ruby -prefix "foo::bar::" example.i

Starting with SWIG 1.3.20, you can also choose to wrap everything into the global module by specifying the `-globalmodule` option on the SWIG command line, i.e.

    $ swig -ruby -globalmodule example.i

Note that this does not relieve you of the requirement of specifying the SWIG module name with the `%module` directive (or the `-module` command-line option) as described earlier.

When choosing a module name, do not use the same name as a built-in Ruby command or standard module name, as the results may be unpredictable. Similarly, if you're using the `-globalmodule` option to wrap everything into the global module, take care that the names of your constants, classes and methods don't conflict with any of Ruby's built-in names.

### 30.3.2 Functions

Global functions are wrapped as Ruby module methods. For example, given the SWIG interface file `example.i`:

    %module example

    int fact(int n);

and C source file `example.c`:

    int fact(int n) {
     if (n == 0)
     return 1;
     return (n * fact(n-1));
    }

SWIG will generate a method fact in the Example module that can be used like so:

    $ irb
    irb(main):001:0> require 'example'
    true
    irb(main):002:0> Example.fact(4)
    24

### 30.3.3 Variable Linking

C/C++ global variables are wrapped as a pair of singleton methods for the module: one to get the value of the global variable and one to set it. For example, the following SWIG interface file declares two global variables:

    // SWIG interface file with global variables
    %module example
    ...
    %inline %{
    extern int variable1;
    extern double Variable2;
    %}
    ...

Now look at the Ruby interface:

    $ irb
    irb(main):001:0> require 'Example'
    true
    irb(main):002:0> Example.variable1 = 2
    2
    irb(main):003:0> Example.Variable2 = 4 * 10.3
    41.2
    irb(main):004:0> Example.Variable2
    41.2

If you make an error in variable assignment, you will receive an error message. For example:

    irb(main):005:0> Example.Variable2 = "hello"
    TypeError: no implicit conversion to float from string
    from (irb):5:in `Variable2='
    from (irb):5

If a variable is declared as `const`, it is wrapped as a read-only variable. Attempts to modify its value will result in an error.

To make ordinary variables read-only, you can also use the `%immutable` directive. For example:

    %immutable;
    %inline %{
    extern char *path;
    %}
    %mutable;

The `%immutable` directive stays in effect until it is explicitly disabled using `%mutable`.

### 30.3.4 Constants

C/C++ constants are wrapped as module constants initialized to the appropriate value. To create a constant, use `#define` or the `%constant` directive. For example:

    #define PI 3.14159
    #define VERSION "1.0"

    %constant int FOO = 42;
    %constant const char *path = "/usr/local";

    const int BAR = 32;

Remember to use the :: operator in Ruby to get at these constant values, e.g.

    $ irb
    irb(main):001:0> require 'Example'
    true
    irb(main):002:0> Example::PI
    3.14159

### 30.3.5 Pointers

"Opaque" pointers to arbitrary C/C++ types (i.e. types that aren't explicitly declared in your SWIG interface file) are wrapped as data objects. So, for example, consider a SWIG interface file containing only the declarations:

    Foo *get_foo();
    void set_foo(Foo *foo);

For this case, the get_foo() method returns an instance of an internally generated Ruby class:

    irb(main):001:0> foo = Example::get_foo()
    #<SWIG::TYPE_p_Foo:0x402b1654>

A `NULL` pointer is always represented by the Ruby `nil` object.

### 30.3.6 Structures

C/C++ structs are wrapped as Ruby classes, with accessor methods (i.e. "getters" and "setters") for all of the struct members. For example, this struct declaration:

    struct Vector {
     double x, y;
    };

gets wrapped as a `Vector` class, with Ruby instance methods `x`, `x=`, `y` and `y=`. These methods can be used to access structure data from Ruby as follows:

    $ irb
    irb(main):001:0> require 'Example'
    true
    irb(main):002:0> f = Example::Vector.new
    #<Example::Vector:0x4020b268>
    irb(main):003:0> f.x = 10
    nil
    irb(main):004:0> f.x
    10.0

Similar access is provided for unions and the public data members of C++ classes.

`const` members of a structure are read-only. Data members can also be forced to be read-only using the `%immutable` directive (in C++, `private` may also be used). For example:

    struct Foo {
     ...
     %immutable;
     int x; /* Read-only members */
     char *name;
     %mutable;
     ...
    };

When `char *` members of a structure are wrapped, the contents are assumed to be dynamically allocated using `malloc` or `new` (depending on whether or not SWIG is run with the `-c++` option). When the structure member is set, the old contents will be released and a new value created. If this is not the behavior you want, you will have to use a typemap (described shortly).

Array members are normally wrapped as read-only. For example, this code:

    struct Foo {
     int x[50];
    };

produces a single accessor function like this:

    int *Foo_x_get(Foo *self) {
     return self->x;
    };

If you want to set an array member, you will need to supply a "memberin" typemap described in the section on typemaps. As a special case, SWIG does generate code to set array members of type `char` (allowing you to store a Ruby string in the structure).

When structure members are wrapped, they are handled as pointers. For example,

    struct Foo {
     ...
    };

    struct Bar {
     Foo f;
    };

generates accessor functions such as this:

    Foo *Bar_f_get(Bar *b) {
     return &b->f;
    }

    void Bar_f_set(Bar *b, Foo *val) {
     b->f = *val;
    }

### 30.3.7 C++ classes

Like structs, C++ classes are wrapped by creating a new Ruby class of the same name with accessor methods for the public class member data. Additionally, public member functions for the class are wrapped as Ruby instance methods, and public static member functions are wrapped as Ruby singleton methods. So, given the C++ class declaration:

    class List {
    public:
     List();
     ~List();
     int search(char *item);
     void insert(char *item);
     void remove(char *item);
     char *get(int n);
     int length;
     static void print(List *l);
    };

SWIG would create a `List` class with:

- instance methods search, insert, remove, and get;
- instance methods length and length= (to get and set the value of the length data member); and,
- a print singleton method for the class.

In Ruby, these functions are used as follows:

    require 'Example'

    l = Example::List.new

    l.insert("Ale")
    l.insert("Stout")
    l.insert("Lager")
    Example.print(l)
    l.length()
    ----- produces the following output
    Lager
    Stout
    Ale
    3

### 30.3.8 C++ Inheritance

The SWIG type-checker is fully aware of C++ inheritance. Therefore, if you have classes like this:

    class Parent {
     ...
    };

    class Child : public Parent {
     ...
    };

those classes are wrapped into a hierarchy of Ruby classes that reflect the same inheritance structure. All of the usual Ruby utility methods work normally:

    irb(main):001:0> c = Child.new
    #<Bar:0x4016efd4>
    irb(main):002:0> c.instance_of? Child
    true
    irb(main):003:0> b.instance_of? Parent
    false
    irb(main):004:0> b.is_a? Child
    true
    irb(main):005:0> b.is_a? Parent
    true
    irb(main):006:0> Child < Parent
    true
    irb(main):007:0> Child > Parent
    false

Furthermore, if you have a function like this:

    void spam(Parent *f);

then the function `spam()` accepts `Parent`* or a pointer to any class derived from `Parent`.

Until recently, the Ruby module for SWIG didn't support multiple inheritance, and this is still the default behavior. This doesn't mean that you can't wrap C++ classes which inherit from multiple base classes; it simply means that only the first base class listed in the class declaration is considered, and any additional base classes are ignored. As an example, consider a SWIG interface file with a declaration like this:

    class Derived : public Base1, public Base2
    {
     ...
    };

For this case, the resulting Ruby class (`Derived`) will only consider `Base1` as its superclass. It won't inherit any of `Base2`'s member functions or data and it won't recognize `Base2` as an "ancestor" of `Derived` (i.e. the is_a? relationship would fail). When SWIG processes this interface file, you'll see a warning message like:

    example.i:5: Warning(802): Warning for Derived: Base Base2 ignored.
    Multiple inheritance is not supported in Ruby.

Starting with SWIG 1.3.20, the Ruby module for SWIG provides limited support for multiple inheritance. Because the approach for dealing with multiple inheritance introduces some limitations, this is an optional feature that you can activate with the `-minherit` command-line option:

    $ swig -c++ -ruby -minherit example.i

Using our previous example, if your SWIG interface file contains a declaration like this:

    class Derived : public Base1, public Base2
    {
     ...
    };

and you run SWIG with the `-minherit` command-line option, then you will end up with a Ruby class `Derived` that appears to "inherit" the member data and functions from both `Base1` and `Base2`. What actually happens is that three different top-level classes are created, with Ruby's `Object` class as their superclass. Each of these classes defines a nested module named `Impl`, and it's in these nested `Impl` modules that the actual instance methods for the classes are defined, i.e.

    class Base1
     module Impl
     # Define Base1 methods here
     end
     include Impl
    end

    class Base2
     module Impl
     # Define Base2 methods here
     end
     include Impl
    end

    class Derived
     module Impl
     include Base1::Impl
     include Base2::Impl
     # Define Derived methods here
     end
     include Impl
    end

Observe that after the nested `Impl` module for a class is defined, it is mixed-in to the class itself. Also observe that the `Derived::Impl` module first mixes-in its base classes' `Impl` modules, thus "inheriting" all of their behavior.

The primary drawback is that, unlike the default mode of operation, neither `Base1` nor `Base2` is a true superclass of `Derived` anymore:

    obj = Derived.new
    obj.is_a? Base1 # this will return false...
    obj.is_a? Base2 # ... and so will this

In most cases, this is not a serious problem since objects of type `Derived` will otherwise behave as though they inherit from both `Base1` and `Base2` (i.e. they exhibit "Duck Typing").

### 30.3.9 C++ Overloaded Functions

C++ overloaded functions, methods, and constructors are mostly supported by SWIG. For example, if you have two functions like this:

    void foo(int);
    void foo(char *c);

You can use them in Ruby in a straightforward manner:

    irb(main):001:0> foo(3) # foo(int)
    irb(main):002:0> foo("Hello") # foo(char *c)

Similarly, if you have a class like this,

    class Foo {
    public:
     Foo();
     Foo(const Foo &);
     ...
    };

you can write Ruby code like this:

    irb(main):001:0> f = Foo.new # Create a Foo
    irb(main):002:0> g = Foo.new(f) # Copy f

Overloading support is not quite as flexible as in C++. Sometimes there are methods that SWIG can't disambiguate. For example:

    void spam(int);
    void spam(short);

or

    void foo(Bar *b);
    void foo(Bar &b);

If declarations such as these appear, you will get a warning message like this:

    example.i:12: Warning(509): Overloaded spam(short) is shadowed by spam(int)
    at example.i:11.

To fix this, you either need to ignore or rename one of the methods. For example:

    %rename(spam_short) spam(short);
    ...
    void spam(int);
    void spam(short); // Accessed as spam_short

or

    %ignore spam(short);
    ...
    void spam(int);
    void spam(short); // Ignored

SWIG resolves overloaded functions and methods using a disambiguation scheme that ranks and sorts declarations according to a set of type-precedence rules. The order in which declarations appear in the input does not matter except in situations where ambiguity arises--in this case, the first declaration takes precedence.

Please refer to the "SWIG and C++" chapter for more information about overloading.

### 30.3.10 C++ Operators

For the most part, overloaded operators are handled automatically by SWIG and do not require any special treatment on your part. So if your class declares an overloaded addition operator, e.g.

    class Complex {
     ...
     Complex operator+(Complex &);
     ...
    };

the resulting Ruby class will also support the addition (+) method correctly.

For cases where SWIG's built-in support is not sufficient, C++ operators can be wrapped using the `%rename` directive (available on SWIG 1.3.10 and later releases). All you need to do is give the operator the name of a valid Ruby identifier. For example:

    %rename(add_complex) operator+(Complex &, Complex &);
    ...
    Complex operator+(Complex &, Complex &);

Now, in Ruby, you can do this:

    a = Example::Complex.new(2, 3)
    b = Example::Complex.new(4, -1)
    c = Example.add_complex(a, b)

More details about wrapping C++ operators into Ruby operators is discussed in the section on operator overloading.

### 30.3.11 C++ namespaces

SWIG is aware of C++ namespaces, but namespace names do not appear in the module nor do namespaces result in a module that is broken up into submodules or packages. For example, if you have a file like this,

    %module example

    namespace foo {
     int fact(int n);
     struct Vector {
     double x,y,z;
     };
    };

it works in Ruby as follows:

    irb(main):001:0> require 'example'
    true
    irb(main):002:0> Example.fact(3)
    6
    irb(main):003:0> v = Example::Vector.new
    #<Example::Vector:0x4016f4d4>
    irb(main):004:0> v.x = 3.4
    3.4
    irb(main):004:0> v.y
    0.0

If your program has more than one namespace, name conflicts (if any) can be resolved using `%rename` For example:

    %rename(Bar_spam) Bar::spam;

    namespace Foo {
     int spam();
    }

    namespace Bar {
     int spam();
    }

If you have more than one namespace and your want to keep their symbols separate, consider wrapping them as separate SWIG modules. For example, make the module name the same as the namespace and create extension modules for each namespace separately. If your program utilizes thousands of small deeply nested namespaces each with identical symbol names, well, then you get what you deserve.

### 30.3.12 C++ templates

C++ templates don't present a huge problem for SWIG. However, in order to create wrappers, you have to tell SWIG to create wrappers for a particular template instantiation. To do this, you use the `%template` directive. For example:

    %module example

    %{
    #include "pair.h"
    %}

    template<class T1, class T2>
    struct pair {
     typedef T1 first_type;
     typedef T2 second_type;
     T1 first;
     T2 second;
     pair();
     pair(const T1&, const T2&);
     ~pair();
    };

    %template(Pairii) pair<int,int>;

In Ruby:

    irb(main):001:0> require 'example'
    true
    irb(main):002:0> p = Example::Pairii.new(3, 4)
    #<Example:Pairii:0x4016f4df>
    irb(main):003:0> p.first
    3
    irb(main):004:0> p.second
    4

On a related note, the standard SWIG library contains a number of modules that provide typemaps for standard C++ library classes (such as `std::pair`, `std::string` and `std::vector`). These library modules don't provide wrappers around the templates themselves, but they do make it convenient for users of your extension module to pass Ruby objects (such as arrays and strings) to wrapped C++ code that expects instances of standard C++ templates. For example, suppose the C++ library you're wrapping has a function that expects a vector of floats:

    %module example

    float sum(const std::vector<float>& values);

Rather than go through the hassle of writing an "in" typemap to convert an array of Ruby numbers into a std::vector<float>, you can just use the `std_vector.i` module from the standard SWIG library:

    %module example

    %include std_vector.i
    float sum(const std::vector<float>& values);

Obviously, there is a lot more to template wrapping than shown in these examples. More details can be found in the SWIG and C++ chapter.

### 30.3.13 C++ Smart Pointers

In certain C++ programs, it is common to use classes that have been wrapped by so-called "smart pointers." Generally, this involves the use of a template class that implements `operator->()` like this:

    template<class T> class SmartPtr {
     ...
     T *operator->();
     ...
    }

Then, if you have a class like this,

    class Foo {
    public:
     int x;
     int bar();
    };

A smart pointer would be used in C++ as follows:

    SmartPtr<Foo> p = CreateFoo(); // Created somehow (not shown)
    ...
    p->x = 3; // Foo::x
    int y = p->bar(); // Foo::bar

To wrap this in Ruby, simply tell SWIG about the `SmartPtr` class and the low-level `Foo` object. Make sure you instantiate `SmartPtr` using `%template` if necessary. For example:

    %module example
    ...
    %template(SmartPtrFoo) SmartPtr<Foo>;
    ...

Now, in Ruby, everything should just "work":

    irb(main):001:0> p = Example::CreateFoo() # Create a smart-pointer somehow
    #<Example::SmartPtrFoo:0x4016f4df>
    irb(main):002:0> p.x = 3 # Foo::x
    3
    irb(main):003:0> p.bar() # Foo::bar

If you ever need to access the underlying pointer returned by `operator->()` itself, simply use the `__deref__()` method. For example:

    irb(main):004:0> f = p.__deref__() # Returns underlying Foo *

### 30.3.14 Cross-Language Polymorphism

SWIG's Ruby module supports cross-language polymorphism (a.k.a. the "directors" feature) similar to that for SWIG's Python module. Rather than duplicate the information presented in the Python chapter, this section just notes the differences that you need to be aware of when using this feature with Ruby.

#### 30.3.14.1 Exception Unrolling

Whenever a C++ director class routes one of its virtual member function calls to a Ruby instance method, there's always the possibility that an exception will be raised in the Ruby code. By default, those exceptions are ignored, which simply means that the exception will be exposed to the Ruby interpreter. If you would like to change this behavior, you can use the `%feature("director:except")` directive to indicate what action should be taken when a Ruby exception is raised. The following code should suffice in most cases:

    %feature("director:except") {
     throw Swig::DirectorMethodException($error);
    }

When this feature is activated, the call to the Ruby instance method is "wrapped" using the `rb_rescue2()` function from Ruby's C API. If any Ruby exception is raised, it will be caught here and a C++ exception is raised in its place.

## 30.4 Naming

Ruby has several common naming conventions. Constants are generally in upper case, module and class names are in camel case and methods are in lower case with underscores. For example:

- MATH::PI is a constant name
- MyClass is a class name
- my_method is a method name

Prior to version 1.3.28, SWIG did not support these Ruby conventions. The only modifications it made to names was to capitalize the first letter of constants (which includes module and class names).

SWIG 1.3.28 introduces the new -autorename command line parameter. When this parameter is specified, SWIG will automatically change constant, class and method names to conform with the standard Ruby naming conventions. For example:

    $ swig -ruby -autorename example.i

To disable renaming use the -noautorename command line option.

Since this change significantly changes the wrapper code generated by SWIG, it is turned off by default in SWIG 1.3.28. However, it is planned to become the default option in future releases.

### 30.4.1 Defining Aliases

It's a fairly common practice in the Ruby built-ins and standard library to provide aliases for method names. For example, Array#size is an alias for Array#length. If you would like to provide an alias for one of your class' instance methods, one approach is to use SWIG's `%extend` directive to add a new method of the aliased name that calls the original function. For example:

    class MyArray {
    public:
     // Construct an empty array
     MyArray();

     // Return the size of this array
     size_t length() const;
    };

    %extend MyArray {
     // MyArray#size is an alias for MyArray#length
     size_t size() const {
     return self->length();
     }
    }

A better solution is to use the `%alias` directive (unique to SWIG's Ruby module). The previous example could then be rewritten as:

    // MyArray#size is an alias for MyArray#length
    %alias MyArray::length "size";

    class MyArray {
    public:
     // Construct an empty array
     MyArray();

     // Return the size of this array
     size_t length() const;
    };

Multiple aliases can be associated with a method by providing a comma-separated list of aliases to the `%alias` directive, e.g.

    %alias MyArray::length "amount,quantity,size";

From an end-user's standpoint, there's no functional difference between these two approaches; i.e. they should get the same result from calling either MyArray#size or MyArray#length. However, when the `%alias` directive is used, SWIG doesn't need to generate all of the wrapper code that's usually associated with added methods like our MyArray::size() example.

Note that the `%alias` directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

### 30.4.2 Predicate Methods

Ruby methods that return a boolean value and end in a question mark are known as predicate methods. Examples of predicate methods in standard Ruby classes include Array#empty? (which returns `true` for an array containing no elements) and Object#instance_of? (which returns `true` if the object is an instance of the specified class). For consistency with Ruby conventions, methods that return boolean values should be marked as predicate methods.

One cumbersome solution to this problem is to rename the method (using SWIG's `%rename` directive) and provide a custom typemap that converts the function's actual return type to Ruby's `true` or `false`. For example:

    %rename("is_it_safe?") is_it_safe();

    %typemap(out) int is_it_safe
     "$result = ($1 != 0) ? Qtrue : Qfalse;";

    int is_it_safe();

A better solution is to use the `%predicate` directive (unique to SWIG's Ruby module) to designate a method as a predicate method. For the previous example, this would look like:

    %predicate is_it_safe();

    int is_it_safe();

This method would be invoked from Ruby code like this:

    irb(main):001:0> Example::is_it_safe?
    true

The `%predicate` directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

### 30.4.3 Bang Methods

Ruby methods that modify an object in-place and end in an exclamation mark are known as bang methods. An example of a bang method is Array#sort! which changes the ordering of items in an array. Contrast this with Array#sort, which returns a copy of the array with the items sorted instead of modifying the original array. For consistency with Ruby conventions, methods that modify objects in place should be marked as bang methods.

Bang methods can be marked using the `%bang` directive which is unique to the Ruby module and was introduced in SWIG 1.3.28. For example:

    %bang sort!(arr);

    int sort(int arr[]);

This method would be invoked from Ruby code like this:

    irb(main):001:0> Example::sort!(arr)

The `%bang` directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

### 30.4.4 Getters and Setters

Often times a C++ library will expose properties through getter and setter methods. For example:

    class Foo {
    	Foo() {}

     int getValue() { return value_; }

     void setValue(int value) { value_ = value; }

    private:
     int value_;
    };

By default, SWIG will expose these methods to Ruby as `get_value` and `set_value.` However, it more natural for these methods to be exposed in Ruby as `value` and `value=.` That allows the methods to be used like this:

    irb(main):001:0> foo = Foo.new()
    irb(main):002:0> foo.value = 5
    irb(main):003:0> puts foo.value

This can be done by using the %rename directive:

    %rename("value") Foo::getValue();
    %rename("value=") Foo::setValue(int value);

## 30.5 Input and output parameters

A common problem in some C programs is handling parameters passed as simple pointers. For example:

    void add(int x, int y, int *result) {
     *result = x + y;
    }
    or
    int sub(int *x, int *y) {
     return *x-*y;
    }

The easiest way to handle these situations is to use the `typemaps.i` file. For example:

    %module Example
    %include "typemaps.i"

    void add(int, int, int *OUTPUT);
    int sub(int *INPUT, int *INPUT);

In Ruby, this allows you to pass simple values. For example:

    a = Example.add(3,4)
    puts a
    7
    b = Example.sub(7,4)
    puts b
    3

Notice how the `INPUT` parameters allow integer values to be passed instead of pointers and how the `OUTPUT` parameter creates a return result.

If you don't want to use the names `INPUT` or `OUTPUT`, use the `%apply` directive. For example:

    %module Example
    %include "typemaps.i"

    %apply int *OUTPUT { int *result };
    %apply int *INPUT { int *x, int *y};

    void add(int x, int y, int *result);
    int sub(int *x, int *y);

If a function mutates one of its parameters like this,

    void negate(int *x) {
     *x = -(*x);
    }

you can use `INOUT` like this:

    %include "typemaps.i"
    ...
    void negate(int *INOUT);

In Ruby, a mutated parameter shows up as a return value. For example:

    a = Example.negate(3)
    print a
    -3

The most common use of these special typemap rules is to handle functions that return more than one value. For example, sometimes a function returns a result as well as a special error code:

    /* send message, return number of bytes sent, success code, and error_code */
    int send_message(char *text, int *success, int *error_code);

To wrap such a function, simply use the `OUTPUT` rule above. For example:

    %module example
    %include "typemaps.i"
    ...
    int send_message(char *, int *OUTPUT, int *OUTPUT);

When used in Ruby, the function will return an array of multiple values.

    bytes, success, error_code = send_message("Hello World")
    if not success
     print "error #{error_code} : in send_message"
    else
     print "Sent", bytes
    end

Another way to access multiple return values is to use the `%apply` rule. In the following example, the parameters rows and columns are related to SWIG as `OUTPUT` values through the use of `%apply`

    %module Example
    %include "typemaps.i"
    %apply int *OUTPUT { int *rows, int *columns };
    ...
    void get_dimensions(Matrix *m, int *rows, int*columns);

In Ruby:

    r, c = Example.get_dimensions(m)

## 30.6 Exception handling

### 30.6.1 Using the %exception directive

The SWIG `%exception` directive can be used to define a user-definable exception handler that can convert C/C++ errors into Ruby exceptions. The chapter on Customization Features contains more details, but suppose you have a C++ class like the following :

    class DoubleArray {
     private:
       int n;
       double *ptr;
     public:
       // Create a new array of fixed size
       DoubleArray(int size) {
         ptr = new double[size];
         n = size;
       }

       // Destroy an array
       ~DoubleArray() {
         delete ptr;
       }

       // Return the length of the array
       int length() {
         return n;
       }

       // Get an array item and perform bounds checking.
       double getitem(int i) {
         if ((i >= 0) && (i < n))
           return ptr[i];
         else
           throw RangeError();
       }

       // Set an array item and perform bounds checking.
       void setitem(int i, double val) {
         if ((i >= 0) && (i < n))
           ptr[i] = val;
         else {
           throw RangeError();
         }
       }
     };

Since several methods in this class can throw an exception for an out-of-bounds access, you might want to catch this in the Ruby extension by writing the following in an interface file:

    %exception {
     try {
       $action
     }
     catch (const RangeError&) {
       static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
       rb_raise(cpperror, "Range error.");
     }
    }

    class DoubleArray {
     ...
    };

The exception handling code is inserted directly into generated wrapper functions. When an exception handler is defined, errors can be caught and used to gracefully raise a Ruby exception instead of forcing the entire program to terminate with an uncaught error.

As shown, the exception handling code will be added to every wrapper function. Because this is somewhat inefficient, you might consider refining the exception handler to only apply to specific methods like this:

    %exception getitem {
     try {
       $action
     }
     catch (const RangeError&) {
       static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
       rb_raise(cpperror, "Range error in getitem.");
     }
    }

    %exception setitem {
     try {
       $action
     }
     catch (const RangeError&) {
       static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
       rb_raise(cpperror, "Range error in setitem.");
     }
    }

In this case, the exception handler is only attached to methods and functions named `getitem` and `setitem`.

Since SWIG's exception handling is user-definable, you are not limited to C++ exception handling. See the chapter on Customization Features for more examples.

### 30.6.2 Raising exceptions

There are three ways to raise exceptions from C++ code to Ruby.

The first way is to use `SWIG_exception(int code, const char *msg)`. The following table shows the mappings from SWIG error codes to Ruby exceptions:

SWIG_MemoryError | rb_eNoMemError
SWIG_IOError | rb_eIOError
SWIG_RuntimeError | rb_eRuntimeError
SWIG_IndexError | rb_eIndexError
SWIG_TypeError | rb_eTypeError
SWIG_DivisionByZero | rb_eZeroDivError
SWIG_OverflowError | rb_eRangeError
SWIG_SyntaxError | rb_eSyntaxError
SWIG_ValueError | rb_eArgError
SWIG_SystemError | rb_eFatal
SWIG_AttributeError | rb_eRuntimeError
SWIG_NullReferenceError | rb_eNullReferenceError*
SWIG_ObjectPreviouslyDeletedError | rb_eObjectPreviouslyDeleted*
SWIG_UnknownError | rb_eRuntimeError
* These error classes are created by SWIG and are not built-in Ruby exception classes

The second way to raise errors is to use `SWIG_Raise(obj, type, desc)`. Obj is a C++ instance of an exception class, type is a string specifying the type of exception (for example, "MyError") and desc is the SWIG description of the exception class. For example:

%raise(SWIG_NewPointerObj(e, SWIGTYPE_p_AssertionFailedException, 0), ":AssertionFailedException", SWIGTYPE_p_AssertionFailedException);

This is useful when you want to pass the current exception object directly to Ruby, particularly when the object is an instance of class marked as an `%exceptionclass` (see the next section for more information).

Last, you can raise an exception by directly calling Ruby's C api. This is done by invoking the `rb_raise()` function. The first argument passed to `rb_raise()` is the exception type. You can raise a custom exception type or one of the built-in Ruby exception types.

### 30.6.3 Exception classes

Starting with SWIG 1.3.28, the Ruby module supports the `%exceptionclass` directive, which is used to identify C++ classes that are used as exceptions. Classes that are marked with the `%exceptionclass` directive are exposed in Ruby as child classes of `rb_eRuntimeError`. This alows C++ exceptions to be directly mapped to Ruby exceptions, providing for a more natural integration between C++ code and Ruby code.

    	%exceptionclass CustomError;

    	%inline %{
    	 	class CustomError { };

    		class Foo {
    	   public:
    		 	void test() { throw CustomError; }
    	  };
    	}

From Ruby you can now call this method like this:

    foo = Foo.new
    begin
      foo.test()
    rescue CustomError => e
      puts "Caught custom error"
    end

For another example look at swig/Examples/ruby/exception_class.

## 30.7 Typemaps

This section describes how you can modify SWIG's default wrapping behavior for various C/C++ datatypes using the `%typemap` directive. This is an advanced topic that assumes familiarity with the Ruby C API as well as the material in the "Typemaps" chapter.

Before proceeding, it should be stressed that typemaps are not a required part of using SWIG---the default wrapping behavior is enough in most cases. Typemaps are only used if you want to change some aspect of the primitive C-Ruby interface.

### 30.7.1 What is a typemap?

A typemap is nothing more than a code generation rule that is attached to a specific C datatype. For example, to convert integers from Ruby to C, you might define a typemap like this:

    %module example

    %typemap(in) int {
     $1 = (int) NUM2INT($input);
     printf("Received an integer : %d\n",$1);
    }

    %inline %{
    extern int fact(int n);
    %}

Typemaps are always associated with some specific aspect of code generation. In this case, the "in" method refers to the conversion of input arguments to C/C++. The datatype `int` is the datatype to which the typemap will be applied. The supplied C code is used to convert values. In this code a number of special variables prefaced by a `$` are used. The `$1` variable is placeholder for a local variable of type `int`. The `$input` variable is the input Ruby object.

When this example is compiled into a Ruby module, the following sample code:

    require 'example'

    puts Example.fact(6)

prints the result:

    Received an integer : 6
    720

In this example, the typemap is applied to all occurrences of the `int` datatype. You can refine this by supplying an optional parameter name. For example:

    %module example

    %typemap(in) int n {
     $1 = (int) NUM2INT($input);
     printf("n = %d\n",$1);
    }

    %inline %{
    extern int fact(int n);
    %}

In this case, the typemap code is only attached to arguments that exactly match "`int n`".

The application of a typemap to specific datatypes and argument names involves more than simple text-matching--typemaps are fully integrated into the SWIG type-system. When you define a typemap for `int`, that typemap applies to `int` and qualified variations such as `const int`. In addition, the typemap system follows `typedef` declarations. For example:

    %typemap(in) int n {
     $1 = (int) NUM2INT($input);
     printf("n = %d\n",$1);
    }

    typedef int Integer;
    extern int fact(Integer n); // Above typemap is applied

However, the matching of `typedef` only occurs in one direction. If you defined a typemap for `Integer`, it is not applied to arguments of type `int`.

Typemaps can also be defined for groups of consecutive arguments. For example:

    %typemap(in) (char *str, int len) {
     $1 = STR2CSTR($input);
     $2 = (int) RSTRING($input)->len;
    };

    int count(char c, char *str, int len);

When a multi-argument typemap is defined, the arguments are always handled as a single Ruby object. This allows the function `count` to be used as follows (notice how the length parameter is omitted):

    puts Example.count('o','Hello World')
    2

### 30.7.2 Ruby typemaps

The previous section illustrated an "in" typemap for converting Ruby objects to C. A variety of different typemap methods are defined by the Ruby module. For example, to convert a C integer back into a Ruby object, you might define an "out" typemap like this:

    %typemap(out) int {
     $result = INT2NUM($1);
    }

The following list details all of the typemap methods that can be used by the Ruby module:

`%typemap(in)`

Converts Ruby objects to input function arguments

`%typemap(out)`

Converts return value of a C function to a Ruby object

`%typemap(varin)`

Assigns a C global variable from a Ruby object

`%typemap(varout)`

Returns a C global variable as a Ruby object

`%typemap(freearg)`

Cleans up a function argument (if necessary)

`%typemap(argout)`

Output argument processing

`%typemap(ret)`

Cleanup of function return values

`%typemap(memberin)`

Setting of structure/class member data

`%typemap(globalin)`

Setting of C global variables

`%typemap(check)`

Checks function input values.

`%typemap(default)`

Set a default value for an argument (making it optional).

`%typemap(arginit)`

Initialize an argument to a value before any conversions occur.

Examples of these typemaps appears in the section on typemap examples

### 30.7.3 Typemap variables

Within a typemap, a number of special variables prefaced with a `$` may appear. A full list of variables can be found in the "Typemaps" chapter. This is a list of the most common variables:

`$1`

A C local variable corresponding to the actual type specified in the `%typemap` directive. For input values, this is a C local variable that is supposed to hold an argument value. For output values, this is the raw result that is supposed to be returned to Ruby.

`$input`

A `VALUE` holding a raw Ruby object with an argument or variable value.

`$result`

A `VALUE` that holds the result to be returned to Ruby.

`$1_name`

The parameter name that was matched.

`$1_type`

The actual C datatype matched by the typemap.

`$1_ltype`

An assignable version of the datatype matched by the typemap (a type that can appear on the left-hand-side of a C assignment operation). This type is stripped of qualifiers and may be an altered version of `$1_type`. All arguments and local variables in wrapper functions are declared using this type so that their values can be properly assigned.

`$symname`

The Ruby name of the wrapper function being created.

### 30.7.4 Useful Functions

When you write a typemap, you usually have to work directly with Ruby objects. The following functions may prove to be useful. (These functions plus many more can be found in Programming Ruby, by David Thomas and Andrew Hunt.)

#### 30.7.4.1 C Datatypes to Ruby Objects

    INT2NUM(long or int) - int to Fixnum or Bignum
    INT2FIX(long or int) - int to Fixnum (faster than INT2NUM)
    CHR2FIX(char) - char to Fixnum
    rb_str_new2(char*) - char* to String
    rb_float_new(double) - double to Float

#### 30.7.4.2 Ruby Objects to C Datatypes

     int NUM2INT(Numeric)
     int FIX2INT(Numeric)
     unsigned int NUM2UINT(Numeric)
     unsigned int FIX2UINT(Numeric)
     long NUM2LONG(Numeric)
     long FIX2LONG(Numeric)
    unsigned long FIX2ULONG(Numeric)
     char NUM2CHR(Numeric or String)
     char * STR2CSTR(String)
     char * rb_str2cstr(String, int*length)
     double NUM2DBL(Numeric)

#### 30.7.4.3 Macros for VALUE

`RSTRING(str)->len`

length of the Ruby string

`RSTRING(str)->ptr`

pointer to string storage

`RARRAY(arr)->len`

length of the Ruby array

`RARRAY(arr)->capa`

capacity of the Ruby array

`RARRAY(arr)->ptr`

pointer to array storage

#### 30.7.4.4 Exceptions

`void rb_raise(VALUE exception, const char *fmt, ...)`

Raises an exception. The given format string fmt and remaining arguments are interpreted as with `printf()`.

`void rb_fatal(const char *fmt, ...)`

Raises a fatal exception, terminating the process. No rescue blocks are called, but ensure blocks will be called. The given format string fmt and remaining arguments are interpreted as with `printf()`.

`void rb_bug(const char *fmt, ...)`

Terminates the process immediately -- no handlers of any sort will be called. The given format string fmt and remaining arguments are interpreted as with `printf()`. You should call this function only if a fatal bug has been exposed.

`void rb_sys_fail(const char *msg)`

Raises a platform-specific exception corresponding to the last known system error, with the given string msg.

`VALUE rb_rescue(VALUE (*body)(VALUE), VALUE args, VALUE(*rescue)(VALUE, VALUE), VALUE rargs)`

Executes body with the given args. If a `StandardError` exception is raised, then execute rescue with the given rargs.

`VALUE rb_ensure(VALUE(*body)(VALUE), VALUE args, VALUE(*ensure)(VALUE), VALUE eargs)`

Executes body with the given args. Whether or not an exception is raised, execute ensure with the given rargs after body has completed.

`VALUE rb_protect(VALUE (*body)(VALUE), VALUE args, int *result)`

Executes body with the given args and returns nonzero in result if any exception was raised.

`void rb_notimplement()`

Raises a `NotImpError` exception to indicate that the enclosed function is not implemented yet, or not available on this platform.

`void rb_exit(int status)`

Exits Ruby with the given status. Raises a `SystemExit` exception and calls registered exit functions and finalizers.

`void rb_warn(const char *fmt, ...)`

Unconditionally issues a warning message to standard error. The given format string fmt and remaining arguments are interpreted as with `printf()`.

`void rb_warning(const char *fmt, ...)`

Conditionally issues a warning message to standard error if Ruby was invoked with the `-w` flag. The given format string fmt and remaining arguments are interpreted as with `printf()`.

#### 30.7.4.5 Iterators

`void rb_iter_break()`

Breaks out of the enclosing iterator block.

`VALUE rb_each(VALUE obj)`

Invokes the `each` method of the given obj.

`VALUE rb_yield(VALUE arg)`

Transfers execution to the iterator block in the current context, passing arg as an argument. Multiple values may be passed in an array.

`int rb_block_given_p()`

Returns `true` if `yield` would execute a block in the current context; that is, if a code block was passed to the current method and is available to be called.

`VALUE rb_iterate(VALUE (*method)(VALUE), VALUE args, VALUE (*block)(VALUE, VALUE), VALUE arg2)`

Invokes method with argument args and block block. A `yield` from that method will invoke block with the argument given to `yield`, and a second argument arg2.

`VALUE rb_catch(const char *tag, VALUE (*proc)(VALUE, VALUE), VALUE value)`

Equivalent to Ruby's `catch`.

`void rb_throw(const char *tag, VALUE value)`

Equivalent to Ruby's `throw`.

### 30.7.5 Typemap Examples

This section includes a few examples of typemaps. For more examples, you might look at the examples in the `Example/ruby` directory.

### 30.7.6 Converting a Ruby array to a char **

A common problem in many C programs is the processing of command line arguments, which are usually passed in an array of `NULL` terminated strings. The following SWIG interface file allows a Ruby Array instance to be used as a `char **` object.

    %module argv

    // This tells SWIG to treat char ** as a special case
    %typemap(in) char ** {
     /* Get the length of the array */
     int size = RARRAY($input)->len;
     int i;
     $1 = (char **) malloc((size+1)*sizeof(char *));
     /* Get the first element in memory */
     VALUE *ptr = RARRAY($input)->ptr;
     for (i=0; i < size; i++, ptr++)
     /* Convert Ruby Object String to char* */
     $1[i]= STR2CSTR(*ptr);
     $1[i]=NULL; /* End of list */
    }

    // This cleans up the char ** array created before
    // the function call

    %typemap(freearg) char ** {
     free((char *) $1);
    }

    // Now a test function
    %inline %{
    int print_args(char **argv) {
     int i = 0;
     while (argv[i]) {
     printf("argv[%d] = %s\n", i,argv[i]);
     i++;
     }
     return i;
    }
    %}

When this module is compiled, the wrapped C function now operates as follows :

    require 'Argv'
    Argv.print_args(["Dave","Mike","Mary","Jane","John"])
    argv[0] = Dave
    argv[1] = Mike
    argv[2] = Mary
    argv[3] = Jane
    argv[4] = John

In the example, two different typemaps are used. The "in" typemap is used to receive an input argument and convert it to a C array. Since dynamic memory allocation is used to allocate memory for the array, the "freearg" typemap is used to later release this memory after the execution of the C function.

### 30.7.7 Collecting arguments in a hash

Ruby's solution to the "keyword arguments" capability of some other languages is to allow the programmer to pass in one or more key-value pairs as arguments to a function. All of those key-value pairs are collected in a single `Hash` argument that's presented to the function. If it makes sense, you might want to provide similar functionality for your Ruby interface. For example, suppose you'd like to wrap this C function that collects information about people's vital statistics:

    void setVitalStats(const char *person, int nattributes, const char **names, int *values);

and you'd like to be able to call it from Ruby by passing in an arbitrary number of key-value pairs as inputs, e.g.

    setVitalStats("Fred",
     'weight' => 270,
    	'age' => 42
    	)

To make this work, you need to write a typemap that expects a Ruby `Hash` as its input and somehow extracts the last three arguments (nattributes, names and values) needed by your C function. Let's start with the basics:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
    }

This `%typemap` directive tells SWIG that we want to match any function declaration that has the specified types and names of arguments somewhere in the argument list. The fact that we specified the argument names (nattributes, names and values) in our typemap is significant; this ensures that SWIG won't try to apply this typemap to other functions it sees that happen to have a similar declaration with different argument names. The arguments that appear in the second set of parentheses (keys_arr, i, key and val) define local variables that our typemap will need.

Since we expect the input argument to be a `Hash`, let's next add a check for that:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
    }

`Check_Type()` is just a macro (defined in the Ruby header files) that confirms that the input argument is of the correct type; if it isn't, an exception will be raised.

The next task is to determine how many key-value pairs are present in the hash; we'll assign this number to the first typemap argument (`$1`). This is a little tricky since the Ruby/C API doesn't provide a public function for querying the size of a hash, but we can get around that by calling the hash's size method directly and converting its result to a C `int` value:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
    }

So now we know the number of attributes. Next we need to initialize the second and third typemap arguments (i.e. the two C arrays) to `NULL` and set the stage for extracting the keys and values from the hash:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
     $2 = NULL;
     $3 = NULL;
     if ($1 > 0) {
     $2 = (char **) malloc($1*sizeof(char *));
     $3 = (int *) malloc($1*sizeof(int));
     }
    }

There are a number of ways we could extract the keys and values from the input hash, but the simplest approach is to first call the hash's keys method (which returns a Ruby array of the keys) and then start looping over the elements in that array:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
     $2 = NULL;
     $3 = NULL;
     if ($1 > 0) {
     $2 = (char **) malloc($1*sizeof(char *));
     $3 = (int *) malloc($1*sizeof(int));
     keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
     for (i = 0; i < $1; i++) {
     }
    }
    }

Recall that keys_arr and i are local variables for this typemap. For each element in the keys_arr array, we want to get the key itself, as well as the value corresponding to that key in the hash:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
     $2 = NULL;
     $3 = NULL;
     if ($1 > 0) {
     $2 = (char **) malloc($1*sizeof(char *));
     $3 = (int *) malloc($1*sizeof(int));
     keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
     for (i = 0; i < $1; i++) {
     key = rb_ary_entry(keys_arr, i);
     val = rb_hash_aref($input, key);
    }
    }
    }

To be safe, we should again use the `Check_Type()` macro to confirm that the key is a `String` and the value is a `Fixnum`:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
     $2 = NULL;
     $3 = NULL;
     if ($1 > 0) {
     $2 = (char **) malloc($1*sizeof(char *));
     $3 = (int *) malloc($1*sizeof(int));
     keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
     for (i = 0; i < $1; i++) {
     key = rb_ary_entry(keys_arr, i);
     val = rb_hash_aref($input, key);
     Check_Type(key, T_STRING);
     Check_Type(val, T_FIXNUM);
    }
    }
    }

Finally, we can convert these Ruby objects into their C equivalents and store them in our local C arrays:

    %typemap(in) (int nattributes, const char **names, const int *values)
     (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
     $2 = NULL;
     $3 = NULL;
     if ($1 > 0) {
     $2 = (char **) malloc($1*sizeof(char *));
     $3 = (int *) malloc($1*sizeof(int));
     keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
     for (i = 0; i < $1; i++) {
     key = rb_ary_entry(keys_arr, i);
     val = rb_hash_aref($input, key);
     Check_Type(key, T_STRING);
     Check_Type(val, T_FIXNUM);
     $2[i] = STR2CSTR(key);
     $3[i] = NUM2INT(val);
    }
    }
    }

We're not done yet. Since we used `malloc()` to dynamically allocate the memory used for the names and values arguments, we need to provide a corresponding "freearg" typemap to free that memory so that there is no memory leak. Fortunately, this typemap is a lot easier to write:

    %typemap(freearg) (int nattributes, const char **names, const int *values) {
     free((void *) $2);
     free((void *) $3);
    }

All of the code for this example, as well as a sample Ruby program that uses the extension, can be found in the `Examples/ruby/hashargs` directory of the SWIG distribution.

### 30.7.8 Pointer handling

Occasionally, it might be necessary to convert pointer values that have been stored using the SWIG typed-pointer representation. Since there are several ways in which pointers can be represented, the following two functions are used to safely perform this conversion:

`int SWIG_ConvertPtr(VALUE obj, void **ptr, swig_type_info *ty, int flags)`

Converts a Ruby object obj to a C pointer whose address is ptr (i.e. ptr is a pointer to a pointer). The third argument, ty, is a pointer to a SWIG type descriptor structure. If ty is not `NULL`, that type information is used to validate type compatibility and other aspects of the type conversion. If flags is non-zero, any type errors encountered during this validation result in a Ruby `TypeError` exception being raised; if flags is zero, such type errors will cause `SWIG_ConvertPtr()` to return -1 but not raise an exception. If ty is `NULL`, no type-checking is performed.

`VALUE SWIG_NewPointerObj(void *ptr, swig_type_info *ty, int own)`

Creates a new Ruby pointer object. Here, ptr is the pointer to convert, ty is the SWIG type descriptor structure that describes the type, and own is a flag that indicates whether or not Ruby should take ownership of the pointer (i.e. whether Ruby should free this data when the corresponding Ruby instance is garbage-collected).

Both of these functions require the use of a special SWIG type-descriptor structure. This structure contains information about the mangled name of the datatype, type-equivalence information, as well as information about converting pointer values under C++ inheritance. For a type of `Foo *`, the type descriptor structure is usually accessed as follows:

    Foo *foo;
    SWIG_ConvertPtr($input, (void **) &foo, SWIGTYPE_p_Foo, 1);

    VALUE obj;
    obj = SWIG_NewPointerObj(f, SWIGTYPE_p_Foo, 0);

In a typemap, the type descriptor should always be accessed using the special typemap variable `$1_descriptor`. For example:

    %typemap(in) Foo * {
     SWIG_ConvertPtr($input, (void **) &$1, $1_descriptor, 1);
    }

#### 30.7.8.1 Ruby Datatype Wrapping

`VALUE Data_Wrap_Struct(VALUE class, void (*mark)(void *), void (*free)(void *), void *ptr)`

Given a pointer ptr to some C data, and the two garbage collection routines for this data (mark and free), return a `VALUE` for the Ruby object.

`VALUE Data_Make_Struct(VALUE class, c-type, void (*mark)(void *), void (*free)(void *), c-type *ptr)`

Allocates a new instance of a C data type c-type, assigns it to the pointer ptr, then wraps that pointer with `Data_Wrap_Struct()` as above.

`Data_Get_Struct(VALUE obj, c-type, c-type *ptr)`

Retrieves the original C pointer of type c-type from the data object obj and assigns that pointer to ptr.

### 30.7.9 Example: STL Vector to Ruby Array

FIXME: This example is out of place here!

Another use for macros and type maps is to create a Ruby array from a STL vector of pointers. In essence, copy of all the pointers in the vector into a Ruby array. The use of the macro is to make the typemap so generic that any vector with pointers can use the type map. The following is an example of how to construct this type of macro/typemap and should give insight into constructing similar typemaps for other STL structures:

    %define PTR_VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)
    %typemap(out) vectorclassname &, const vectorclassname & {
     VALUE arr = rb_ary_new2($1->size());
     vectorclassname::iterator i = $1->begin(), iend = $1->end();
     for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));
     $result = arr;
    }
    %typemap(out) vectorclassname, const vectorclassname {
     VALUE arr = rb_ary_new2($1.size());
     vectorclassname::iterator i = $1.begin(), iend = $1.end();
     for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));
     $result = arr;
    }
    %enddef

Note, that the "`c ## classname.klass"` is used in the preprocessor step to determine the actual object from the class name.

To use the macro with a class Foo, the following is used:

    PTR_VECTOR_TO_RUBY_ARRAY(vector<foo *="">, Foo)

It is also possible to create a STL vector of Ruby objects:

    %define RUBY_ARRAY_TO_PTR_VECTOR(vectorclassname, classname)
    %typemap(in) vectorclassname &, const vectorclassname & {
     Check_Type($input, T_ARRAY);
     vectorclassname *vec = new vectorclassname;
     int len = RARRAY($input)->len;
     for (int i=0; i!=len; i++) {
     VALUE inst = rb_ary_entry($input, i);
     //The following _should_ work but doesn't on HPUX
     // Check_Type(inst, T_DATA);
     classname *element = NULL;
     Data_Get_Struct(inst, classname, element);
     vec->push_back(element);
     }
     $1 = vec;
    }

    %typemap(freearg) vectorclassname &, const vectorclassname & {
     delete $1;
    }
    %enddef

It is also possible to create a Ruby array from a vector of static data types:

    %define VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)
    %typemap(out) vectorclassname &, const vectorclassname & {
     VALUE arr = rb_ary_new2($1->size());
     vectorclassname::iterator i = $1->begin(), iend = $1->end();
     for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &(*i)));
     $result = arr;
    }
    %typemap(out) vectorclassname, const vectorclassname {
     VALUE arr = rb_ary_new2($1.size());
     vectorclassname::iterator i = $1.begin(), iend = $1.end();
     for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &(*i)));
     $result = arr;
    }
    %enddef

## 30.8 Advanced Topics

### 30.8.1 Operator overloading

SWIG allows operator overloading with, by using the `%extend` or `%rename` commands in SWIG and the following operator names (derived from Python):

     General
    __repr__ - inspect
    __str__ - to_s
    __cmp__ - <=>
    __hash__ - hash
    __nonzero__ - nonzero?

     Callable
    __call__ - call

     Collection
    __len__ - length
    __getitem__ - []
    __setitem__ - []=

     Numeric
    __add__ - +
    __sub__ - -
    __mul__ - *
    __div__ - /
    __mod__ - %
    __divmod__ - divmod
    __pow__ - **
    __lshift__ - <<
    __rshift__ - >>
    __and__ - &
    __xor__ - ^
    __or__ - |
    __neg__ - -@
    __pos__ - +@
    __abs__ - abs
    __invert__ - ~
    __int__ - to_i
    __float__ - to_f
    __coerce__ - coerce

    Additions in 1.3.13
    __lt__ - <
    __le__ - <=
    __eq__ - ==
    __gt__ - >
    __ge__ - >=

Note that although SWIG supports the `__eq__` magic method name for defining an equivalence operator, there is no separate method for handling inequality since Ruby parses the expression a != b as !(a == b).

### 30.8.2 Creating Multi-Module Packages

The chapter on Working with Modules discusses the basics of creating multi-module extensions with SWIG, and in particular the considerations for sharing runtime type information among the different modules.

As an example, consider one module's interface file (`shape.i`) that defines our base class:

    %module shape

    %{
    #include "Shape.h"
    %}

    class Shape {
    protected:
     double xpos;
     double ypos;
    protected:
     Shape(double x, double y);
    public:
     double getX() const;
     double getY() const;
    };

We also have a separate interface file (`circle.i`) that defines a derived class:

    %module circle

    %{
    #include "Shape.h"
    #include "Circle.h"
    %}

    // Import the base class definition from Shape module
    %import shape.i

    class Circle : public Shape {
    protected:
     double radius;
    public:
     Circle(double x, double y, double r);
     double getRadius() const;
    };

We'll start by building the Shape extension module:

    $ swig -c++ -ruby shape.i

SWIG generates a wrapper file named `shape_wrap.cxx`. To compile this into a dynamically loadable extension for Ruby, prepare an `extconf.rb` script using this template:

    require 'mkmf'

    # Since the SWIG runtime support library for Ruby
    # depends on the Ruby library, make sure it's in the list
    # of libraries.
    $libs = append_library($libs, Config::CONFIG['RUBY_INSTALL_NAME'])

    # Create the makefile
    create_makefile('shape')

Run this script to create a `Makefile` and then type `make` to build the shared library:

    $ ruby extconf.rb
    creating Makefile
    $ make
    g++ -fPIC -g -O2 -I. -I/usr/local/lib/ruby/1.7/i686-linux \
    -I. -c shape_wrap.cxx
    gcc -shared -L/usr/local/lib -o shape.so shape_wrap.o -L. \
    -lruby -lruby -lc

Note that depending on your installation, the outputs may be slightly different; these outputs are those for a Linux-based development environment. The end result should be a shared library (here, `shape.so`) containing the extension module code. Now repeat this process in a separate directory for the Circle module:

1. Run SWIG to generate the wrapper code (`circle_wrap.cxx`);
2. Write an `extconf.rb` script that your end-users can use to create a platform-specific `Makefile` for the extension;
3. Build the shared library for this extension by typing `make`.

Once you've built both of these extension modules, you can test them interactively in IRB to confirm that the `Shape` and `Circle` modules are properly loaded and initialized:

    $ irb
    irb(main):001:0> require 'shape'
    true
    irb(main):002:0> require 'circle'
    true
    irb(main):003:0> c = Circle::Circle.new(5, 5, 20)
    #<Circle::Circle:0xa097208>
    irb(main):004:0> c.kind_of? Shape::Shape
    true
    irb(main):005:0> c.getX()
    5.0

### 30.8.3 Specifying Mixin Modules

The Ruby language doesn't support multiple inheritance, but it does allow you to mix one or more modules into a class using Ruby's `include` method. For example, if you have a Ruby class that defines an each instance method, e.g.

    class Set
     def initialize
     @members = []
     end

     def each
     @members.each { |m| yield m }
     end
    end

then you can mix-in Ruby's `Enumerable` module to easily add a lot of functionality to your class:

    class Set
     include Enumerable
    def initialize
    @members = []
    end
    def each
    @members.each { |m| yield m }
    end
    end

To get the same benefit for your SWIG-wrapped classes, you can use the `%mixin` directive to specify the names of one or more modules that should be mixed-in to a class. For the above example, the SWIG interface specification might look like this:

    %mixin Set "Enumerable";

    class Set {
    public:
     // Constructor
     Set();

     // Iterates through set members
     void each();
    };

Multiple modules can be mixed into a class by providing a comma-separated list of module names to the `%mixin` directive, e.g.

    %mixin Set "Fee,Fi,Fo,Fum";

Note that the `%mixin` directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

## 30.9 Memory Management

One of the most common issues in generating SWIG bindings for Ruby is proper memory management. The key to proper memory management is clearly defining whether a wrapper Ruby object owns the underlying C struct or C++ class. There are two possibilities:

- The Ruby object is responsible for freeing the C struct or C++ object
- The Ruby object should not free the C struct or C++ object because it will be freed by the underlying C or C++ code

To complicate matters, object ownership may transfer from Ruby to C++ (or vice versa) depending on what function or methods are invoked. Clearly, developing a SWIG wrapper requires a thorough understanding of how the underlying library manages memory.

### 30.9.1 Mark and Sweep Garbage Collector

Ruby uses a mark and sweep garbage collector. When the garbage collector runs, it finds all the "root" objects, including local variables, global variables, global constants, hardware registers and the C stack. For each root object, the garbage collector sets its mark flag to true and calls `rb_gc_mark` on the object. The job of `rb_gc_mark` is to recursively mark all the objects that a Ruby object has a reference to (ignoring those objects that have already been marked). Those objects, in turn, may reference other objects. This process will continue until all active objects have been "marked." After the mark phase comes the sweep phase. In the sweep phase, all objects that have not been marked will be garbage collected. For more information about the Ruby garbage collector please refer to http://rubygarden.org/ruby/ruby?GCAndExtensions.

The Ruby C/API provides extension developers two hooks into the garbage collector - a "mark" function and a "sweep" function. By default these functions are set to NULL.

If a C struct or C++ class references any other Ruby objects, then it must provide a "mark" function. The "mark" function should identify any referenced Ruby objects by calling the rb_gc_mark function for each one. Unsurprisingly, this function will be called by the Ruby garbage during the "mark" phase.

During the sweep phase, Ruby destroys any unused objects. If any memory has been allocated in creating the underlying C struct or C++ struct, then a "free" function must be defined that deallocates this memory.

### 30.9.2 Object Ownership

As described above, memory management depends on clearly defining who is responsible for freeing the underlying C struct or C++ class. If the Ruby object is responsible for freeing the C++ object, then a "free" function must be registered for the object. If the Ruby object is not responsible for freeing the underlying memory, then a "free" function must not be registered for the object.

For the most part, SWIG takes care of memory management issues. The rules it uses are:

- When calling a C++ object's constructor from Ruby, SWIG will assign a "free" function thereby making the Ruby object responsible for freeing the C++ object
- When calling a C++ member function that returns a pointer, SWIG will not assign a "free" function thereby making the underlying library responsible for freeing the object.

To make this clearer, let's look at an example. Assume we have a Foo and a Bar class.

    /* File "RubyOwernshipExample.h" */

    class Foo
    {
    public:
     Foo() {}
     ~Foo() {}
    };

    class Bar
    {
     Foo *foo_;
    public:
     Bar(): foo_(new Foo) {}
     ~Bar() { delete foo_; }
     Foo* get_foo() { return foo_; }
     Foo* get_new_foo() { return new Foo; }
     void set_foo(Foo *foo) { delete foo_; foo_ = foo; }
    };

First, consider this Ruby code:

    foo = Foo.new

In this case, the Ruby code calls the underlying `Foo` C++ constructor, thus creating a new `foo` object. By default, SWIG will assign the new Ruby object a "free" function. When the Ruby object is garbage collected, the "free" function will be called. It in turn will call `Foo's` destructor.

Next, consider this code:

    bar = Bar.new
    foo = bar.get_foo()

In this case, the Ruby code calls a C++ member function, `get_foo`. By default, SWIG will not assign the Ruby object a "free" function. Thus, when the Ruby object is garbage collected the underlying C++ `foo` object is not affected.

Unfortunately, the real world is not as simple as the examples above. For example:

    bar = Bar.new
    foo = bar.get_new_foo()

In this case, the default SWIG behavior for calling member functions is incorrect. The Ruby object should assume ownership of the returned object. This can be done by using the %newobject directive. See Object ownership and %newobject for more information.

The SWIG default mappings are also incorrect in this case:

    foo = Foo.new
    bar = Bar.new
    bar.set_foo(foo)

Without modification, this code will cause a segmentation fault. When the Ruby `foo` object goes out of scope, it will free the underlying C++ `foo` object. However, when the Ruby bar object goes out of scope, it will call the C++ bar destructor which will also free the C++ `foo` object. The problem is that object ownership is transferred from the Ruby object to the C++ object when the `set_foo` method is called. This can be done by using the special DISOWN type map, which was added to the Ruby bindings in SWIG-1.3.26.

Thus, a correct SWIG interface file correct mapping for these classes is:

    /* File RubyOwnershipExample.i */

    %module RubyOwnershipExample

    %{
    #include "RubyOwnershipExample.h"
    %}

    class Foo
    {
    public:
     Foo();
     ~Foo();
    };

    class Bar
    {
     Foo *foo_;
    public:
     Bar();
     ~Bar();
     Foo* get_foo();

     %newobject get_new_foo;
     Foo* get_new_foo();

     %apply SWIGTYPE *DISOWN {Foo *foo};
     void set_foo(Foo *foo);
     %clear Foo *foo;
    };

This code can be seen in swig/examples/ruby/tracking.

### 30.9.3 Object Tracking

The remaining parts of this section will use the class library shown below to illustrate different memory management techniques. The class library models a zoo and the animals it contains.

    %module zoo

    %{
    #include <string>
    #include <vector>

    #include "zoo.h"
    %}

    class Animal
    {
    private:
     typedef std::vector<Animal*> AnimalsType;
     typedef AnimalsType::iterator IterType;
    protected:
     AnimalsType animals;
    protected:
     std::string name_;
    public:
     // Construct an animal with this name
     Animal(const char* name) : name_(name) {}

     // Return the animal's name
     const char* get_name() const { return name.c_str(); }
    };

    class Zoo
    {
    protected:
     std::vector<animal *=""> animals;

    public:
     // Construct an empty zoo
     Zoo() {}

     /* Create a new animal. */
     static Animal* Zoo::create_animal(const char* name)
     {
     return new Animal(name);
     }

     // Add a new animal to the zoo
     void add_animal(Animal* animal) {
     animals.push_back(animal);
     }

     Animal* remove_animal(size_t i) {
     Animal* result = this->animals[i];
     IterType iter = this->animals.begin();
     std::advance(iter, i);
     this->animals.erase(iter);

     return result;
     }

     // Return the number of animals in the zoo
     size_t get_num_animals() const {
     return animals.size();
     }

     // Return a pointer to the ith animal
     Animal* get_animal(size_t i) const {
     return animals[i];
     }
    };

Let's say you SWIG this code and then run IRB:

    $ irb
    irb(main):001:0> require 'example'
    => true

    irb(main):002:0> tiger1 = Example::Animal.new("tiger1")
    => #<Example::Animal:0x2be3820>

    irb(main):004:0> tiger1.get_name()
    => "tiger1"

    irb(main):003:0> zoo = Example::Zoo.new()
    => #<Example::Zoo:0x2be0a60>

    irb(main):006:0> zoo.add_animal(tiger)
    => nil

    irb(main):007:0> zoo.get_num_animals()
    => 1

    irb(main):007:0> tiger2 = zoo.remove_animal(0)
    => #<Example::Animal:0x2bd4a18>

    irb(main):008:0> tiger2.get_name()
    => "tiger1"

    irb(main):009:0> tiger1.equal?(tiger2)
    => false

Pay particular attention to the code `tiger1.equal?(tiger2)`. Note that the two Ruby objects are not the same - but they reference the same underlying C++ object. This can cause problems. For example:

    irb(main):010:0> tiger1 = nil
    => nil

    irb(main):011:0> GC.start
    => nil

    irb(main):012:0> tiger2.get_name()
    (irb):12: [BUG] Segmentation fault

After the the garbage collector runs, as a result of our call to `GC.start`, calling`tiger2.get_name()` causes a segmentation fault. The problem is that when `tiger1` is garbage collected, it frees the underlying C++ object. Thus, when `tiger2` calls the `get_name()` method it invokes it on a destroyed object.

This problem can be avoided if SWIG enforces a one-to-one mapping between Ruby objects and C++ classes. This can be done via the use of the `%trackobjects` functionality available in SWIG-1.3.26. and later.

When the `%trackobjects` is turned on, SWIG automatically keeps track of mappings between C++ objects and Ruby objects. Note that enabling object tracking causes a slight performance degradation. Test results show this degradation to be about 3% to 5% when creating and destroying 100,000 animals in a row.

Since `%trackobjects` is implemented as a `%feature`, it uses the same name matching rules as other kinds of features (see the chapter on "Customization Features") . Thus it can be applied on a class-by-class basis if needed. To fix the example above:

    %module example

    %{
    #include "example.h"
    %}

    /* Tell SWIG that create_animal creates a new object */
    %newobject Zoo::create_animal;

    /* Tell SWIG to keep track of mappings between C/C++ structs/classes. */
    %trackobjects;

    %include "example.h"

When this code runs we see:

    $ irb
    irb(main):001:0> require 'example'
    => true

    irb(main):002:0> tiger1 = Example::Animal.new("tiger1")
    => #<Example::Animal:0x2be37d8>

    irb(main):003:0> zoo = Example::Zoo.new()
    => #<Example::Zoo:0x2be0a18>

    irb(main):004:0> zoo.add_animal(tiger1)
    => nil

    irb(main):006:0> tiger2 = zoo.remove_animal(0)
    => #<Example::Animal:0x2be37d8>

    irb(main):007:0> tiger1.equal?(tiger2)
    => true

    irb(main):008:0> tiger1 = nil
    => nil

    irb(main):009:0> GC.start
    => nil

    irb(main):010:0> tiger.get_name()
    => "tiger1"
    irb(main):011:0>

For those who are interested, object tracking is implemented by storing Ruby objects in a hash table and keying them on C++ pointers. The underlying API is:

    static void SWIG_RubyAddTracking(void* ptr, VALUE object);
    static VALUE SWIG_RubyInstanceFor(void* ptr) ;
    static void SWIG_RubyRemoveTracking(void* ptr);
    static void SWIG_RubyUnlinkObjects(void* ptr);

When an object is created, SWIG will automatically call the `SWIG_RubyAddTracking` method. Similarly, when an object is deleted, SWIG will call the `SWIG_RubyRemoveTracking`. When an object is returned to Ruby from C++, SWIG will use the `SWIG_RubyInstanceFor` method to ensure a one-to-one mapping from Ruby to C++ objects. Last, the `RubyUnlinkObjects` method unlinks a Ruby object from its underlying C++ object.

In general, you will only need to use the `SWIG_RubyInstanceFor`, which is required for implementing mark functions as shown below. However, if you implement your own free functions (see below) you may also have to call the`SWIG_RubyRemoveTracking` and `RubyUnlinkObjects` methods.

### 30.9.4 Mark Functions

With a bit more testing, we see that our class library still has problems. For example:

    $ irb
    irb(main):001:0> require 'example'
    => true

    irb(main):002:0> tiger1 = Example::Animal.new("tiger1")
    => #<Example::Animal:0x2bea6a8>

    irb(main):003:0> zoo = Example::Zoo.new()
    => #<Example::Zoo:0x2be7960>

    irb(main):004:0> zoo.add_animal(tiger1)
    => nil

    irb(main):007:0> tiger1 = nil
    => nil

    irb(main):007:0> GC.start
    => nil

    irb(main):005:0> tiger2 = zoo.get_animal(0)
    (irb):12: [BUG] Segmentation fault

The problem is that Ruby does not know that the `zoo` object contains a reference to a Ruby object. Thus, when Ruby garbage collects tiger1 it frees the underlying C++ object.

This can be fixed by implementing a `mark` function as described above in the Mark and Sweep Garbage Collector section. You can specify a mark function by using the `%markfunc` directive. Since the `%markfunc` directive is implemented using SWIG's' "features" mechanism it uses the same name matching rules as other kinds of features (see the chapter on "Customization Features" for more details).

A `mark` function takes a single argument, which is a pointer to the C++ object being marked; it should, in turn, call `rb_gc_mark()` for any instances that are reachable from the current object. The mark function for our `Zoo` class should therefore loop over all of the C++ animal objects in the zoo object, look up their Ruby object equivalent, and then call `rb_gc_mark()`. One possible implementation is:

    %module example

    %{
    #include "example.h"
    %}

    /* Keep track of mappings between C/C++ structs/classes
     and Ruby objects so we can implement a mark function. */
    %trackobjects;

    /* Specify the mark function */
    %markfunc Zoo "mark_Zoo";

    %include "example.h"

    %header %{

    static void mark_Zoo(void* ptr) {
     Zoo* zoo = (Zoo*) ptr;

     /* Loop over each object and tell the garbage collector
     that we are holding a reference to them. */
     int count = zoo->get_num_animals();

     for(int i = 0; i < count; ++i) {
       Animal* animal = zoo->get_animal(i);
       VALUE object = SWIG_RubyInstanceFor(animal);

       if (object != Qnil) {
         rb_gc_mark(object);
       }
     }
    }
    %}

Note the `mark` function is dependent on the `SWIG_RUBY_InstanceFor` method, and thus requires that `%trackobjects` is enabled. For more information, please refer to the track_object.i test case in the SWIG test suite.

When this code is compiled we now see:

    $ irb
    irb(main):002:0> tiger1=Example::Animal.new("tiger1")
    => #<Example::Animal:0x2be3bf8>

    irb(main):003:0> Example::Zoo.new()
    => #<Example::Zoo:0x2be1780>

    irb(main):004:0> zoo = Example::Zoo.new()
    => #<Example::Zoo:0x2bde9c0>

    irb(main):005:0> zoo.add_animal(tiger1)
    => nil

    irb(main):009:0> tiger1 = nil
    => nil

    irb(main):010:0> GC.start
    => nil
    irb(main):014:0> tiger2 = zoo.get_animal(0)
    => #<Example::Animal:0x2be3bf8>

    irb(main):015:0> tiger2.get_name()
    => "tiger1"
    irb(main):016:0>

This code can be seen in swig/examples/ruby/mark_function.

### 30.9.5 Free Functions

By default, SWIG creates a "free" function that is called when a Ruby object is garbage collected. The free function simply calls the C++ object's destructor.

However, sometimes an appropriate destructor does not exist or special processing needs to be performed before the destructor is called. Therefore, SWIG allows you to manually specify a "free" function via the use of the `%freefunc` directive. The `%freefunc` directive is implemented using SWIG's' "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

IMPORTANT ! - If you define your own free function, then you must ensure that you call the underlying C++ object's destructor. In addition, if object tracking is activated for the object's class, you must also call the `SWIG_RubyRemoveTracking` function (of course call this before you destroy the C++ object). Note that it is harmless to call this method if object tracking if off so it is advised to always call it.

Note there is a subtle interaction between object ownership and free functions. A custom defined free function will only be called if the Ruby object owns the underlying C++ object. This also to Ruby objects which are created, but then transfer ownership to C++ objects via the use of the `disown` typemap described above.

To show how to use the `%freefunc` directive, let's slightly change our example. Assume that the zoo object is responsible for freeing animal that it contains. This means that the Zoo::add_animal function should be marked with a DISOWN typemap and the destructor should be updated as below::

    Zoo::~Zoo() {
     IterType iter = this->animals.begin();
     IterType end = this->animals.end();

     for(iter; iter != end; ++iter) {
     Animal* animal = *iter;
     delete animal;
     }
    }

When we use these objects in IRB we see:

    $irb
    irb(main):002:0> require 'example'
    => true

    irb(main):003:0> zoo = Example::Zoo.new()
    => #<Example::Zoo:0x2be0fe8>

    irb(main):005:0> tiger1 = Example::Animal.new("tiger1")
    => #<Example::Animal:0x2bda760>

    irb(main):006:0> zoo.add_animal(tiger1)
    => nil

    irb(main):007:0> zoo = nil
    => nil

    irb(main):008:0> GC.start
    => nil

    irb(main):009:0> tiger1.get_name()
    (irb):12: [BUG] Segmentation fault

The error happens because the C++ `animal` object is freed when the `zoo` object is freed. Although this error is unavoidable, we can at least prevent the segmentation fault. To do this requires enabling object tracking and implementing a custom free function that calls the `SWIG_RubyUnlinkObjects` function for each animal object that is destroyed. The `SWIG_RubyUnlinkObjects` function notifies SWIG that a Ruby object's underlying C++ object is no longer valid. Once notified, SWIG will intercept any calls from the existing Ruby object to the destroyed C++ object and raise an exception.

    %module example

    %{
    #include "example.h"
    %}

    /* Specify that ownership is transferred to the zoo
    	when calling add_animal */
    %apply SWIGTYPE *DISOWN { Animal* animal };

    /* Track objects */
    %trackobjects;

    /* Specify the mark function */
    %freefunc Zoo "free_Zoo";

    %include "example.h"

    %header %{
     static void free_Zoo(void* ptr) {
     Zoo* zoo = (Zoo*) ptr;

     /* Loop over each animal */
     int count = zoo->get_num_animals();

     for(int i = 0; i < count; ++i) {
     /* Get an animal */
     Animal* animal = zoo->get_animal(i);

     /* Unlink the Ruby object from the C++ object */
     SWIG_RubyUnlinkObjects(animal);

     /* Now remove the tracking for this animal */
     SWIG_RubyRemoveTracking(animal);
     }

     /* Now call SWIG_RemoveMapping for the zoo */
     SWIG_RemoveMapping(ptr);

     /* Now free the zoo which will free the animals it contains */
     delete zoo;
     }
    %}

Now when we use these objects in IRB we see:

    $irb
    irb(main):002:0> require 'example'
    => true

    irb(main):003:0> zoo = Example::Zoo.new()
    => #<Example::Zoo:0x2be0fe8>

    irb(main):005:0> tiger1 = Example::Animal.new("tiger1")
    => #<Example::Animal:0x2bda760>

    irb(main):006:0> zoo.add_animal(tiger1)
    => nil

    irb(main):007:0> zoo = nil
    => nil

    irb(main):008:0> GC.start
    => nil

    irb(main):009:0> tiger1.get_name()
    RuntimeError: This Animal * already released
     from (irb):10:in `get_name'
     from (irb):10
    irb(main):011:0>

Notice that SWIG can now detect the underlying C++ object has been freed, and thus raises a runtime exception.

This code can be seen in swig/examples/ruby/free_function.