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Olly Betts de7ed84f77 Recommend compiling with PIC consistently. 
While shared objects with non-PIC code work on some architectures
(notably x86), unless code is always PIC on that arch (not true for x86)
doing so requires runtime relocations, which prevents the object
actually being shared, and means such segments can't be marked as
read-only.
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>SWIG and Perl5</title>
<link rel="stylesheet" type="text/css" href="style.css">
</head>
<body bgcolor="#ffffff">
<H1><a name="Perl5"></a>31 SWIG and Perl5</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Perl5_nn2">Overview</a>
<li><a href="#Perl5_nn3">Preliminaries</a>
<ul>
<li><a href="#Perl5_nn4">Getting the right header files</a>
<li><a href="#Perl5_nn5">Compiling a dynamic module</a>
<li><a href="#Perl5_nn6">Building a dynamic module with MakeMaker</a>
<li><a href="#Perl5_nn7">Building a static version of Perl</a>
<li><a href="#Perl5_nn8">Using the module</a>
<li><a href="#Perl5_nn9">Compilation problems and compiling with C++</a>
<li><a href="#Perl5_nn10">Compiling for 64-bit platforms</a>
</ul>
<li><a href="#Perl5_nn11">Building Perl Extensions under Windows</a>
<ul>
<li><a href="#Perl5_nn12">Running SWIG from Developer Studio</a>
<li><a href="#Perl5_nn13">Using other compilers</a>
</ul>
<li><a href="#Perl5_nn14">The low-level interface</a>
<ul>
<li><a href="#Perl5_nn15">Functions</a>
<li><a href="#Perl5_nn16">Global variables</a>
<li><a href="#Perl5_nn17">Constants</a>
<li><a href="#Perl5_nn18">Pointers</a>
<li><a href="#Perl5_nn19">Structures</a>
<li><a href="#Perl5_nn20">C++ classes</a>
<li><a href="#Perl5_nn21">C++ classes and type-checking</a>
<li><a href="#Perl5_nn22">C++ overloaded functions</a>
<li><a href="#Perl5_nn23">Operators</a>
<li><a href="#Perl5_nn24">Modules and packages</a>
</ul>
<li><a href="#Perl5_nn25">Input and output parameters</a>
<li><a href="#Perl5_nn26">Exception handling</a>
<li><a href="#Perl5_nn27">Remapping datatypes with typemaps</a>
<ul>
<li><a href="#Perl5_nn28">A simple typemap example</a>
<li><a href="#Perl5_nn29">Perl5 typemaps</a>
<li><a href="#Perl5_nn30">Typemap variables</a>
<li><a href="#Perl5_nn31">Useful functions</a>
</ul>
<li><a href="#Perl5_nn32">Typemap Examples</a>
<ul>
<li><a href="#Perl5_nn33">Converting a Perl5 array to a char **</a>
<li><a href="#Perl5_nn34">Return values</a>
<li><a href="#Perl5_nn35">Returning values from arguments</a>
<li><a href="#Perl5_nn36">Accessing array structure members</a>
<li><a href="#Perl5_nn37">Turning Perl references into C pointers</a>
<li><a href="#Perl5_nn38">Pointer handling</a>
</ul>
<li><a href="#Perl5_nn39">Proxy classes</a>
<ul>
<li><a href="#Perl5_nn40">Preliminaries</a>
<li><a href="#Perl5_nn41">Structure and class wrappers</a>
<li><a href="#Perl5_nn42">Object Ownership</a>
<li><a href="#Perl5_nn43">Nested Objects</a>
<li><a href="#Perl5_nn44">Proxy Functions</a>
<li><a href="#Perl5_nn45">Inheritance</a>
<li><a href="#Perl5_nn46">Modifying the proxy methods</a>
</ul>
<li><a href="#Perl5_nn47">Adding additional Perl code</a>
<li><a href="#Perl5_directors">Cross language polymorphism</a>
<ul>
<li><a href="#Perl5_nn48">Enabling directors</a>
<li><a href="#Perl5_nn49">Director classes</a>
<li><a href="#Perl5_nn50">Ownership and object destruction</a>
<li><a href="#Perl5_nn51">Exception unrolling</a>
<li><a href="#Perl5_nn52">Overhead and code bloat</a>
<li><a href="#Perl5_nn53">Typemaps</a>
</ul>
</ul>
</div>
<!-- INDEX -->
<p>
<b>Caution: This chapter is under repair!</b>
</p>
<p>
This chapter describes SWIG's support of Perl5. Although the Perl5
module is one of the earliest SWIG modules, it has continued to evolve
and has been improved greatly with the help of SWIG users. For the
best results, it is recommended that SWIG be used with Perl 5.8 or
later. We're no longer testing regularly with older versions, but
Perl 5.6 seems to mostly work, while older versions don't.
</p>
<H2><a name="Perl5_nn2"></a>31.1 Overview</H2>
<p>
To build Perl extension modules, SWIG uses a layered approach. At
the lowest level, simple procedural wrappers are generated for
functions, classes, methods, and other declarations in the input file.
Then, for structures and classes, an optional collection of Perl
proxy classes can be generated in order to provide a more natural object oriented Perl
interface. These proxy classes simply build upon the low-level interface.
</p>
<p>
In describing the Perl interface, this chapter begins by covering the
essentials. First, the problem of configuration, compiling,
and installing Perl modules is discussed. Next, the low-level
procedural interface is presented. Finally, proxy classes are
described. Advanced customization features, typemaps, and other
options are found near the end of the chapter.
</p>
<H2><a name="Perl5_nn3"></a>31.2 Preliminaries</H2>
<p>
To build a Perl5 module, run SWIG using the <tt>-perl</tt> option as
follows:
</p>
<div class="code"><pre>
swig -perl example.i
</pre></div>
<p>
This produces two files. The first file, <tt>example_wrap.c</tt>
contains all of the C code needed to build a Perl5 module. The second
file, <tt>example.pm</tt> contains supporting Perl code needed to
properly load the module.
</p>
<p>
To build the module, you will need to compile the file
<tt>example_wrap.c</tt> and link it with the rest of your program.
</p>
<H3><a name="Perl5_nn4"></a>31.2.1 Getting the right header files</H3>
<p>
In order to compile, SWIG extensions need the following Perl5 header files:</p>
<div class="code"><pre>
#include "Extern.h"
#include "perl.h"
#include "XSUB.h"
</pre></div>
<p>
These are typically located in a directory like this</p>
<div class="code"><pre>
/usr/lib/perl/5.14/CORE
</pre></div>
<p>
The SWIG configuration script automatically tries to locate this directory so
that it can compile examples. However, if you need to find out where the directory is
located, an easy way to find out is to ask Perl itself:
</p>
<div class="code">
<pre>
$ perl -e 'use Config; print "$Config{archlib}\n";'
/usr/lib/perl/5.14
</pre>
</div>
<H3><a name="Perl5_nn5"></a>31.2.2 Compiling a dynamic module</H3>
<p>
The preferred approach to building an extension module is to compile it into
a shared object file or DLL. Assuming you have code you need to link to in a file called <tt>example.c</tt>,
you will need to compile your program using commands like this (shown for Linux):
</p>
<div class="code"><pre>
$ swig -perl example.i
$ gcc -fPIC example.c
$ gcc -fPIC -c example_wrap.c -I/usr/lib/perl/5.14/CORE -Dbool=char
$ gcc -shared example.o example_wrap.o -o example.so
</pre></div>
<p>
The exact compiler options vary from platform to platform.
SWIG tries to guess the right options when it is installed. Therefore,
you may want to start with one of the examples in the <tt>SWIG/Examples/perl5</tt>
directory. If that doesn't work, you will need to read the man-pages for
your compiler and linker to get the right set of options. You might also
check the <a href="http://www.dabeaz.com/cgi-bin/wiki.pl">SWIG Wiki</a> for
additional information.
</p>
<p>
When linking the module, the name of the shared object file must match the module name used in
the SWIG interface file. If you used `<tt>%module example</tt>', then
the target should be named `<tt>example.so</tt>',
`<tt>example.sl</tt>', or the appropriate dynamic module name on your system.
</p>
<H3><a name="Perl5_nn6"></a>31.2.3 Building a dynamic module with MakeMaker</H3>
<p>
It is also possible to use Perl to build dynamically loadable modules
for you using the MakeMaker utility. To do this, write a Perl
script such as the following:</p>
<div class="targetlang"><pre>
# File : Makefile.PL
use ExtUtils::MakeMaker;
WriteMakefile(
`NAME' =&gt; `example', # Name of package
`LIBS' =&gt; [`-lm'], # Name of custom libraries
`OBJECT' =&gt; `example.o example_wrap.o' # Object files
);
</pre></div>
<p>
Now, to build a module, simply follow these steps:</p>
<div class="code"><pre>
$ perl Makefile.PL
$ make
$ make install
</pre></div>
<p>
If you are planning to distribute a SWIG-generated module, this is
the preferred approach to compilation. More information about MakeMaker can be
found in "Programming Perl, 2nd ed." by Larry Wall, Tom Christiansen,
and Randal Schwartz.</p>
<H3><a name="Perl5_nn7"></a>31.2.4 Building a static version of Perl</H3>
<p>
If you machine does not support dynamic loading or if you've tried to
use it without success, you can build a new version of the Perl
interpreter with your SWIG extensions added to it. To build a static
extension, you first need to invoke SWIG as follows:</p>
<div class="code"><pre>
$ swig -perl -static example.i
</pre></div>
<p>
By default SWIG includes code for dynamic loading, but the
<tt>-static</tt> option takes it out.</p>
<p>
Next, you will need to supply a <tt>main()</tt> function that
initializes your extension and starts the Perl interpreter. While,
this may sound daunting, SWIG can do this for you automatically as
follows:</p>
<div class="targetlang"><pre>
%module example
%inline %{
extern double My_variable;
extern int fact(int);
%}
// Include code for rebuilding Perl
%include &lt;perlmain.i&gt;
</pre></div>
<p>
The same thing can be accomplished by running SWIG as follows:</p>
<div class="code"><pre>
$ swig -perl -static -lperlmain.i example.i
</pre></div>
<p>
The <tt>perlmain.i</tt> file inserts Perl's <tt>main()</tt> function
into the wrapper code and automatically initializes the SWIG generated
module. If you just want to make a quick a dirty module, this may be
the easiest way. By default, the <tt>perlmain.i</tt> code does not
initialize any other Perl extensions. If you need to use other
packages, you will need to modify it appropriately. You can do this by
just copying <tt>perlmain.i</tt> out of the SWIG library, placing it
in your own directory, and modifying it to suit your purposes.</p>
<p>
To build your new Perl executable, follow the exact same procedure as
for a dynamic module, but change the link line to something like this:
</p>
<div class="code"><pre>
$ gcc example.o example_wrap.o -L/usr/lib/perl/5.14/CORE \
-lperl -lsocket -lnsl -lm -o myperl
</pre></div>
<p>
This will produce a new version of Perl called <tt>myperl</tt>. It
should be functionality identical to Perl with your C/C++ extension
added to it. Depending on your machine, you may need to link with
additional libraries such as <tt>-lsocket, -lnsl, -ldl</tt>, etc.
</p>
<H3><a name="Perl5_nn8"></a>31.2.5 Using the module</H3>
<p>
To use the module, simply use the Perl <tt>use</tt> statement. If
all goes well, you will be able to do this:
</p>
<div class="targetlang"><pre>
$ perl
use example;
print example::fact(4),"\n";
24
</pre></div>
<p>
A common error received by first-time users is the following:
</p>
<div class="targetlang">
<pre>
use example;
Can't locate example.pm in @INC (@INC contains: /etc/perl /usr/local/lib/perl/5.14.2 /usr/local/share/perl/5.14.2 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.14 /usr/share/perl/5.14 /usr/local/lib/site_perl .) at - line 1.
BEGIN failed--compilation aborted at - line 1.
</pre>
</div>
<p>
This error is almost caused when the name of the shared object file you created doesn't match the module name
you specified with the <tt>%module</tt> directive.
</p>
<p>
A somewhat related, but slightly different error is this:
</p>
<div class="targetlang">
<pre>
use example;
Can't find 'boot_example' symbol in ./example.so
at - line 1
BEGIN failed--compilation aborted at - line 1.
</pre>
</div>
<p>
This error is generated because Perl can't locate the module bootstrap function in the
SWIG extension module. This could be caused by a mismatch between the module name and the shared library name.
However, another possible cause is forgetting to link the SWIG-generated wrapper code with the rest
of your application when you linked the extension module.
</p>
<p>
Another common error is the following:
</p>
<div class="targetlang">
<pre>
use example;
Can't load './example.so' for module example: ./example.so:
undefined symbol: Foo at /usr/lib/perl/5.14/i386-linux/DynaLoader.pm line 169.
at - line 1
BEGIN failed--compilation aborted at - line 1.
</pre>
</div>
<p>
This error usually indicates that you forgot to include some object
files or libraries in the linking of the shared library file. Make
sure you compile both the SWIG wrapper file and your original program
into a shared library file. Make sure you pass all of the required libraries
to the linker.
</p>
<p>
Sometimes unresolved symbols occur because a wrapper has been created
for a function that doesn't actually exist in a library. This usually
occurs when a header file includes a declaration for a function that
was never actually implemented or it was removed from a library
without updating the header file. To fix this, you can either edit
the SWIG input file to remove the offending declaration or you can use
the <tt>%ignore</tt> directive to ignore the declaration. Better yet,
update the header file so that it doesn't have an undefined declaration.
</p>
<p>
Finally, suppose that your extension module is linked with another library like this:
</p>
<div class="code">
<pre>
$ gcc -shared example.o example_wrap.o -L/home/beazley/projects/lib -lfoo \
-o example.so
</pre>
</div>
<p>
If the <tt>foo</tt> library is compiled as a shared library, you might get the following
error when you try to use your module:
</p>
<div class="targetlang">
<pre>
use example;
Can't load './example.so' for module example: libfoo.so: cannot open shared object file:
No such file or directory at /usr/lib/perl/5.14/i386-linux/DynaLoader.pm line 169.
at - line 1
BEGIN failed--compilation aborted at - line 1.
&gt;&gt;&gt;
</pre>
</div>
<p>
This error is generated because the dynamic linker can't locate the
<tt>libfoo.so</tt> library. When shared libraries are loaded, the
system normally only checks a few standard locations such as
<tt>/usr/lib</tt> and <tt>/usr/local/lib</tt>. To get the loader to look in other
locations, there are several things you can do. First, you can recompile your extension
module with extra path information. For example, on Linux you can do this:
</p>
<div class="code">
<pre>
$ gcc -shared example.o example_wrap.o -L/home/beazley/projects/lib -lfoo \
<b>-Xlinker -rpath /home/beazley/projects/lib \</b>
-o example.so
</pre>
</div>
<p>
Alternatively, you can set the <tt>LD_LIBRARY_PATH</tt> environment
variable to include the directory with your shared libraries. If
setting <tt>LD_LIBRARY_PATH</tt>, be aware that setting this variable
can introduce a noticeable performance impact on all other
applications that you run. To set it only for Perl, you might want
to do this instead:
</p>
<div class="code">
<pre>
$ env LD_LIBRARY_PATH=/home/beazley/projects/lib perl
</pre>
</div>
<p>
Finally, you can use a command such as <tt>ldconfig</tt> (Linux) or
<tt>crle</tt> (Solaris) to add additional search paths to the default
system configuration (this requires root access and you will need to
read the man pages).
</p>
<H3><a name="Perl5_nn9"></a>31.2.6 Compilation problems and compiling with C++</H3>
<p>
Compilation of C++ extensions has traditionally been a tricky problem.
Since the Perl interpreter is written in C, you need to take steps to
make sure C++ is properly initialized and that modules are compiled
correctly.
</p>
<p>
On most machines, C++ extension modules should be linked using the C++
compiler. For example:
</p>
<div class="code"><pre>
$ swig -c++ -perl example.i
$ g++ -fPIC -c example.cxx
$ g++ -fPIC -c example_wrap.cxx -I/usr/lib/perl/5.14/i386-linux/CORE
$ <b>g++ -shared example.o example_wrap.o -o example.so</b>
</pre></div>
<p>
In addition to this, you may need to include additional library
files to make it work. For example, if you are using the Sun C++ compiler on
Solaris, you often need to add an extra library <tt>-lCrun</tt> like this:
</p>
<div class="code"><pre>
$ swig -c++ -perl example.i
$ CC -Kpic -c example.cxx
$ CC -Kpic -c example_wrap.cxx -I/usr/lib/perl/5.14/i386-linux/CORE
$ CC -shared example.o example_wrap.o -o example.so <b>-lCrun</b>
</pre></div>
<p>
Of course, the names of the extra libraries are completely non-portable---you will
probably need to do some experimentation.
</p>
<p>
Another possible compile problem comes from recent versions of Perl (5.8.0) and the GNU tools.
If you see errors having to do with _crypt_struct, that means _GNU_SOURCE is not defined and
it needs to be. So you should compile the wrapper like:
</p>
<div class="code"><pre>
$ g++ -fPIC -c example_wrap.cxx -I/usr/lib/perl/5.8.0/CORE -D_GNU_SOURCE
</pre></div>
<p>
-D_GNU_SOURCE is also included in the Perl ccflags, which can be found by running
</p>
<div class="code"><pre>
$ perl -e 'use Config; print "$Config{ccflags}\n";'
</pre></div>
<p>
So you could also compile the wrapper like
</p>
<div class="code"><pre>
$ g++ -fPIC -c example_wrap.cxx -I/usr/lib/perl/5.8.0/CORE \
`perl -MConfig -e 'print $Config{ccflags}'`
</pre></div>
<p>
Sometimes people have suggested that it is necessary to relink the
Perl interpreter using the C++ compiler to make C++ extension modules work.
In the experience of this author, this has never actually appeared to be
necessary on most platforms. Relinking the interpreter with C++ really only includes the
special run-time libraries described above---as long as you link your extension
modules with these libraries, it should not be necessary to rebuild Perl.
</p>
<p>
If you aren't entirely sure about the linking of a C++ extension, you
might look at an existing C++ program. On many Unix machines, the
<tt>ldd</tt> command will list library dependencies. This should give
you some clues about what you might have to include when you link your
extension module. For example, notice the first line of output here:
</p>
<div class="code">
<pre>
$ ldd swig
<b>libstdc++-libc6.1-1.so.2 =&gt; /usr/lib/libstdc++-libc6.1-1.so.2 (0x40019000)</b>
libm.so.6 =&gt; /lib/libm.so.6 (0x4005b000)
libc.so.6 =&gt; /lib/libc.so.6 (0x40077000)
/lib/ld-linux.so.2 =&gt; /lib/ld-linux.so.2 (0x40000000)
$
</pre>
</div>
<p>
If linking wasn't enough of a problem, another major complication of C++ is that it does not
define any sort of standard for binary linking of libraries. This
means that C++ code compiled by different compilers will not link
together properly as libraries nor is the memory layout of classes and
data structures implemented in any kind of portable manner. In a
monolithic C++ program, this problem may be unnoticed. However, in Perl, it
is possible for different extension modules to be compiled with
different C++ compilers. As long as these modules are self-contained,
this probably won't matter. However, if these modules start sharing data,
you will need to take steps to avoid segmentation faults and other
erratic program behavior. Also, be aware that certain C++ features, especially RTTI,
can behave strangely when working with multiple modules.
</p>
<p>
It should be noted that you may get a lot of error messages
about the '<tt>bool</tt>' datatype when compiling a C++ Perl module. If
you experience this problem, you can try the following:</p>
<ul>
<li>Use <tt>-DHAS_BOOL</tt> when compiling the SWIG wrapper code
<li>Or use <tt>-Dbool=char</tt> when compiling.
</ul>
<p>
Finally, recent versions of Perl (5.8.0) have namespace conflict problems. Perl defines a bunch
of short macros to make the Perl API function names shorter. For example, in
/usr/lib/perl/5.8.0/CORE/embed.h there is a line:
</p>
<div class="code"><pre>
#define do_open Perl_do_open
</pre></div>
<p>
The problem is, in the &lt;iostream&gt; header from GNU libstdc++v3 there is a private
function named do_open. If &lt;iostream&gt; is included after the perl headers, then
the Perl macro causes the iostream do_open to be renamed, which causes compile errors.
Hopefully in the future Perl will support a PERL_NO_SHORT_NAMES flag, but for now the
only solution is to undef the macros that conflict. Lib/perl5/noembed.h in the SWIG
source has a list of macros that are known to conflict with either standard headers or
other headers. But if you get macro type conflicts from other macros not included
in Lib/perl5/noembed.h while compiling the wrapper, you will
have to find the macro that conflicts and add an #undef into the .i file. Please report
any conflicting macros you find to <a href="http://www.swig.org/mail.html">swig-user mailing list</a>.
</p>
<H3><a name="Perl5_nn10"></a>31.2.7 Compiling for 64-bit platforms</H3>
<p>
On platforms that support 64-bit applications (Solaris, Irix, etc.),
special care is required when building extension modules. On these
machines, 64-bit applications are compiled and linked using a different
set of compiler/linker options. In addition, it is not generally possible to mix
32-bit and 64-bit code together in the same application.
</p>
<p>
To utilize 64-bits, the Perl executable will need to be recompiled
as a 64-bit application. In addition, all libraries, wrapper code,
and every other part of your application will need to be compiled for
64-bits. If you plan to use other third-party extension modules, they
will also have to be recompiled as 64-bit extensions.
</p>
<p>
If you are wrapping commercial software for which you have no source
code, you will be forced to use the same linking standard as used by
that software. This may prevent the use of 64-bit extensions. It may
also introduce problems on platforms that support more than one
linking standard (e.g., -o32 and -n32 on Irix).
</p>
<H2><a name="Perl5_nn11"></a>31.3 Building Perl Extensions under Windows</H2>
<p>
Building a SWIG extension to Perl under Windows is roughly
similar to the process used with Unix. Normally, you will want to
produce a DLL that can be loaded into the Perl interpreter. This
section assumes you are using SWIG with Microsoft Visual C++
although the procedure may be similar with other compilers.
</p>
<H3><a name="Perl5_nn12"></a>31.3.1 Running SWIG from Developer Studio</H3>
<p>
If you are developing your application within Microsoft developer
studio, SWIG can be invoked as a custom build option. The process
roughly requires these steps:</p>
<ul>
<li>Open up a new workspace and use the AppWizard to select a DLL
project.
<li>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
(ie. <tt>example_wrap.c</tt>). Note: If using C++, choose a
different suffix for the wrapper file such as
<tt>example_wrap.cxx</tt>. Don't worry if the wrapper file doesn't
exist yet--Developer studio will keep a reference to it around.
<li>Select the SWIG interface file and go to the settings menu. Under
settings, select the "Custom Build" option.
<li>Enter "SWIG" in the description field.
<li>Enter "<tt>swig -perl5 -o $(ProjDir)\$(InputName)_wrap.cxx
$(InputPath)</tt>" in the "Build command(s) field"
<li>Enter "<tt>$(ProjDir)\$(InputName)_wrap.c</tt>xx" in the "Output
files(s) field".
<li>Next, select the settings for the entire project and go to
"C++:Preprocessor". Add the include directories for your Perl 5
installation under "Additional include directories".
<li>Define the symbols WIN32 and MSWIN32 under preprocessor options.
If using the ActiveWare port, also define the symbol PERL_OBJECT.
Note that all extensions to the ActiveWare port must be compiled with
the C++ compiler since Perl has been encapsulated in a C++ class.
<li>Finally, select the settings for the entire project and go to
"Link Options". Add the Perl library file to your link libraries.
For example "perl.lib". Also, set the name of the output file to
match the name of your Perl module (ie. example.dll).
<li>Build your project.
</ul>
<p>
Now, assuming you made it this far, 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 Perl extension, simply run Perl and
use the use command as normal. For example:
</p>
<div class="targetlang"><pre>
DOS &gt; perl
use example;
$a = example::fact(4);
print "$a\n";
</pre></div>
<H3><a name="Perl5_nn13"></a>31.3.2 Using other compilers</H3>
<p>
SWIG is known to work with Cygwin and may work with other compilers on Windows.
For general hints and suggestions refer to the <a href="Windows.html#Windows">Windows</a> chapter.
</p>
<H2><a name="Perl5_nn14"></a>31.4 The low-level interface</H2>
<p>
At its core, the Perl module uses a simple low-level interface
to C function, variables, constants, and classes. This low-level interface
can be used to control your application. However, it is also used to
construct more user-friendly proxy classes as described in the next section.
</p>
<H3><a name="Perl5_nn15"></a>31.4.1 Functions</H3>
<p>
C functions are converted into new Perl built-in commands (or
subroutines). For example:
</p>
<div class="targetlang"><pre>
%module example
int fact(int a);
...
</pre></div>
<p>
Now, in Perl:
</p>
<div class="targetlang"><pre>
use example;
$a = &amp;example::fact(2);
</pre></div>
<H3><a name="Perl5_nn16"></a>31.4.2 Global variables</H3>
<p>
Global variables are handled using Perl's magic
variable mechanism. SWIG generates a pair of functions
that intercept read/write operations and attaches them to a Perl variable with
the same name as the C global variable. Thus, an interface like this </p>
<div class="targetlang"><pre>
%module example;
...
double Spam;
...
</pre></div>
<p>
is accessed as follows:</p>
<div class="targetlang"><pre>
use example;
print $example::Spam,"\n";
$example::Spam = $example::Spam + 4
# ... etc ...
</pre></div>
<p>
If a variable is declared as <tt>const</tt>, it is wrapped as a
read-only variable. Attempts to modify its value will result in an
error.
</p>
<p>
To make ordinary variables read-only, you can also use the <tt>%immutable</tt> directive. For example:
</p>
<div class="code">
<pre>
%{
extern char *path;
%}
%immutable;
extern char *path;
%mutable;
</pre>
</div>
<p>
The <tt>%immutable</tt> directive stays in effect until it is explicitly disabled or cleared using
<tt>%mutable</tt>.
See the <a href="SWIG.html#SWIG_readonly_variables">Creating read-only variables</a> section for further details.
</p>
<p>
It is also possible to tag a specific variable as read-only like this:
</p>
<div class="code">
<pre>
%{
extern char *path;
%}
%immutable path;
...
...
extern char *path; // Declared later in the input
</pre>
</div>
<H3><a name="Perl5_nn17"></a>31.4.3 Constants</H3>
<p>
By default, constants are wrapped as read-only Perl variables. For example:
</p>
<div class="code">
<pre>
%module example
#define FOO 42
</pre>
</div>
<p>
In Perl:
</p>
<div class="targetlang">
<pre>
use example;
print $example::FOO,"\n"; # OK
$example::FOO = 2; # Error
</pre>
</div>
<p>
Alternatively, if you use swig's <tt>-const</tt> option, constants are wrapped
such that the leading $ isn't required (by using a constant subroutine), which
usually gives a more natural Perl interface, for example:
</p>
<div class="targetlang">
<pre>
use example;
print example::FOO,"\n";
</pre>
</div>
<H3><a name="Perl5_nn18"></a>31.4.4 Pointers</H3>
<p>
SWIG represents pointers as blessed references. A blessed reference
is the same as a Perl reference except that it has additional
information attached to it indicating what kind of reference it
is. That is, if you have a C declaration like this:</p>
<div class="code"><pre>
Matrix *new_Matrix(int n, int m);
</pre></div>
<p>
The module returns a value generated as follows:
</p>
<div class="targetlang"><pre>
$ptr = new_Matrix(int n, int m); # Save pointer return result
bless $ptr, "p_Matrix"; # Bless it as a pointer to Matrix
</pre></div>
<p>
SWIG uses the "blessing" to check the datatype of various pointers.
In the event of a mismatch, an error or warning message is
generated.</p>
<p>
To check to see if a value is the NULL pointer, use the
<tt>defined()</tt> command:</p>
<div class="targetlang"><pre>
if (defined($ptr)) {
print "Not a NULL pointer.";
} else {
print "Is a NULL pointer.";
}
</pre></div>
<p>
To create a NULL pointer, you should pass the <tt>undef</tt> value to
a function.
</p>
<p>
The "value" of a Perl reference is not the same as the underlying C
pointer that SWIG wrapper functions return. Suppose that <tt>$a</tt>
and <tt>$b</tt> are two references that point to the same C object.
In general, <tt>$a</tt> and <tt>$b</tt> will be different--since they
are different references. Thus, it is a mistake to check the equality
of <tt>$a</tt> and <tt>$b</tt> to check the equality of two C
pointers. The correct method to check equality of C pointers is to
dereference them as follows:
</p>
<div class="targetlang"><pre>
if ($$a == $$b) {
print "a and b point to the same thing in C";
} else {
print "a and b point to different objects.";
}
</pre></div>
<p>
As much as you might be inclined to modify a pointer value directly
from Perl, don't. Manipulating pointer values is architecture dependent and
could cause your program to crash. Similarly, don't try to manually cast
a pointer to a new type by reblessing a pointer. This
may not work like you expect and it is particularly dangerous when
casting C++ objects. If you need to cast a pointer or
change its value, consider writing some helper functions instead. For
example:
</p>
<div class="code">
<pre>
%inline %{
/* C-style cast */
Bar *FooToBar(Foo *f) {
return (Bar *) f;
}
/* C++-style cast */
Foo *BarToFoo(Bar *b) {
return dynamic_cast&lt;Foo*&gt;(b);
}
Foo *IncrFoo(Foo *f, int i) {
return f+i;
}
%}
</pre>
</div>
<p>
Also, if working with C++, you should always try
to use the new C++ style casts. For example, in the above code, the
C-style cast may return a bogus result whereas as the C++-style cast will return
<tt>NULL</tt> if the conversion can't be performed.
</p>
<p>
<b>Compatibility Note:</b> In earlier versions, SWIG tried to preserve the same pointer naming conventions
as XS and <tt>xsubpp</tt>. Given the advancement of the SWIG typesystem and the growing differences between
SWIG and XS, this is no longer supported.
</p>
<H3><a name="Perl5_nn19"></a>31.4.5 Structures</H3>
<p>
Access to the contents of a structure are provided through a set of low-level
accessor functions as described in the "SWIG Basics" chapter. For example,
</p>
<div class="code"><pre>
struct Vector {
double x,y,z;
};
</pre></div>
<p>
gets mapped into the following collection of accessor functions:
</p>
<div class="code"><pre>
struct Vector *new_Vector();
void delete_Vector(Vector *v);
double Vector_x_get(Vector *obj)
void Vector_x_set(Vector *obj, double x)
double Vector_y_get(Vector *obj)
void Vector_y_set(Vector *obj, double y)
double Vector_z_get(Vector *obj)
void Vector_z_set(Vector *obj, double z)
</pre></div>
<p>
These functions are then used to access structure data from Perl as follows:
</p>
<div class="targetlang"><pre>
$v = example::new_Vector();
print example::Vector_x_get($v),"\n"; # Get x component
example::Vector_x_set($v,7.8); # Change x component
</pre></div>
<p>
Similar access is provided for unions and the data members of C++ classes.
</p>
<p>
<tt>const</tt> members of a structure are read-only. Data members
can also be forced to be read-only using the <tt>%immutable</tt> directive. For example:
</p>
<div class="code">
<pre>
struct Foo {
...
%immutable;
int x; /* Read-only members */
char *name;
%mutable;
...
};
</pre>
</div>
<p>
When <tt>char *</tt> members of a structure are wrapped, the contents are assumed to be
dynamically allocated using <tt>malloc</tt> or <tt>new</tt> (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 later).
</p>
<p>
Array members are normally wrapped as read-only. For example,
</p>
<div class="code">
<pre>
struct Foo {
int x[50];
};
</pre>
</div>
<p>
produces a single accessor function like this:
</p>
<div class="code">
<pre>
int *Foo_x_get(Foo *self) {
return self-&gt;x;
};
</pre>
</div>
<p>
If you want to set an array member, you will need to supply a "memberin" typemap
described later in this chapter. As a special case, SWIG does generate
code to set array members of type <tt>char</tt> (allowing you to store a Python
string in the structure).
</p>
<p>
When structure members are wrapped, they are handled as pointers. For example,
</p>
<div class="code">
<pre>
struct Foo {
...
};
struct Bar {
Foo f;
};
</pre>
</div>
<p>
generates accessor functions such as this:
</p>
<div class="code">
<pre>
Foo *Bar_f_get(Bar *b) {
return &amp;b-&gt;f;
}
void Bar_f_set(Bar *b, Foo *val) {
b-&gt;f = *val;
}
</pre>
</div>
<H3><a name="Perl5_nn20"></a>31.4.6 C++ classes</H3>
<p>
C++ classes are wrapped by building a set of low level accessor functions.
Consider the following class:
</p>
<div class="code"><pre>
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);
};
</pre></div>
<p>
When wrapped by SWIG, the following functions are created:
</p>
<div class="code"><pre>
List *new_List();
void delete_List(List *l);
int List_search(List *l, char *item);
void List_insert(List *l, char *item);
void List_remove(List *l, char *item);
char *List_get(List *l, int n);
int List_length_get(List *l);
void List_length_set(List *l, int n);
void List_print(List *l);
</pre></div>
<p>
In Perl, these functions are used in a straightforward manner:
</p>
<div class="targetlang"><pre>
use example;
$l = example::new_List();
example::List_insert($l,"Ale");
example::List_insert($l,"Stout");
example::List_insert($l,"Lager")
example::List_print($l)
Lager
Stout
Ale
print example::List_length_get($l),"\n";
3
</pre></div>
<p>
At this low level, C++ objects are really just typed pointers. Member
functions are accessed by calling a C-like wrapper with an instance pointer
as the first argument. Although this interface is fairly primitive, it
provides direct access to C++ objects. A higher level interface using Perl proxy classes
can be built using these low-level accessors. This is described shortly.
</p>
<H3><a name="Perl5_nn21"></a>31.4.7 C++ classes and type-checking</H3>
<p>
The SWIG type-checker is fully aware of C++ inheritance. Therefore, if you have
classes like this
</p>
<div class="code">
<pre>
class Foo {
...
};
class Bar : public Foo {
...
};
</pre>
</div>
<p>
and a function
</p>
<div class="code">
<pre>
void spam(Foo *f);
</pre>
</div>
<p>
then the function <tt>spam()</tt> accepts <tt>Foo *</tt> or a pointer to any class derived from <tt>Foo</tt>.
If necessary, the type-checker also adjusts the value of the pointer (as is necessary when
multiple inheritance is used).
</p>
<H3><a name="Perl5_nn22"></a>31.4.8 C++ overloaded functions</H3>
<p>
If you have a C++ program with overloaded functions or methods, you will need to disambiguate
those methods using <tt>%rename</tt>. For example:
</p>
<div class="code">
<pre>
/* Forward renaming declarations */
%rename(foo_i) foo(int);
%rename(foo_d) foo(double);
...
void foo(int); // Becomes 'foo_i'
void foo(char *c); // Stays 'foo' (not renamed)
class Spam {
public:
void foo(int); // Becomes 'foo_i'
void foo(double); // Becomes 'foo_d'
...
};
</pre>
</div>
<p>
Now, in Perl, the methods are accessed as follows:
</p>
<div class="targetlang">
<pre>
use example;
example::foo_i(3);
$s = example::new_Spam();
example::Spam_foo_i($s,3);
example::Spam_foo_d($s,3.14);
</pre>
</div>
<p>
Please refer to the "SWIG Basics" chapter for more information.
</p>
<H3><a name="Perl5_nn23"></a>31.4.9 Operators</H3>
<p>
As of version 1.3.27 SWIG automatically renames the most common C++ operators, and maps them into the perl module with the proper 'use overload ...' so you don't need to do any work.
</p>
<p>
The following C++ operators are currently supported by the Perl module:
</p>
<ul>
<li>operator++ </li>
<li>operator-- </li>
<li>operator+ </li>
<li>operator- </li>
<li>operator* </li>
<li>operator/ </li>
<li>operator== </li>
<li>operator!= </li>
<li>operator% </li>
<li>operator&gt; </li>
<li>operator&lt; </li>
<li>operator and </li>
<li>operator or </li>
</ul>
<H3><a name="Perl5_nn24"></a>31.4.10 Modules and packages</H3>
<p>
When you create a SWIG extension, everything gets placed into
a single Perl module. The name of the module is determined by the
<tt>%module</tt> directive. To use the module, do the following:
</p>
<div class="targetlang"><pre>
$ perl5
use example; # load the example module
print example::fact(4),"\n" # Call a function in it
24
</pre></div>
<p>
Usually, a module consists of a collection of code that is contained
within a single file. A package, on the other hand, is the Perl
equivalent of a namespace. A package is a lot like a module, except
that it is independent of files. Any number of files may be part of
the same package--or a package may be broken up into a collection of
modules if you prefer to think about it in this way.
</p>
<p>
SWIG installs its functions into a package with the same name as
the module. </p>
<p>
<b>Incompatible Change:</b> previous versions of SWIG enabled you to
change the name of the package by using the -package option, this
feature has been removed in order to properly support modules that
used nested namespaces, e.g. Foo::Bar::Baz. To give your module a
nested namespace simply provide the fully qualified name in your
%module directive: </p>
<div class="code"><pre>
%module "Foo::Bar::Baz"
</pre></div>
<p>
<b>NOTE:</b> the double quotes are necessary.
</p>
<p>
Using the <tt>package</tt> option of the <tt>%module</tt> directive allows
you to specify what Perl namespace that the module will be living in when
installed. This is useful in the situation where a module maintainer
wants to split a large module into smaller pieces to make maintenance
easier, but doesn't want to have that affect the module name used by
applications. So for example, if I wanted to split <tt>XML::Xerces</tt>
into <tt>XML::Xerces::SAX</tt>, etc. , but I wanted all the applications
to be able to access the classes using the <tt>XML::Xerces</tt> namespace
I could use:
</p>
<div class="code">
<pre>
%module(package="XML::Xerces") "XML::Xerces::SAX
</pre>
</div>
<p>
And now all the applications could use the class
<tt>XML::Xerces::SAXParser</tt>. Without the <tt>package</tt> directive
splitting the module would force applications to use the class
<tt>XML::Xerces::SAX::SAXParser</tt>. This could break compatibility for
existing applications that are already using the class under the name
<tt>XML::Xerces::SAXParser</tt>.
</p>
<!--
<p>
This can be changed by giving SWIG the -package
option:
</p>
<div class="code"><pre>
$ swig -perl -package Foo example.i
</pre></div>
<p>
In this case, you still create a module called `<tt>example</tt>' exactly as before, but
all of the functions in that module will be installed into the package
`<tt>Foo</tt>.' For example:
</p>
<div class="targetlang"><pre>
use example; # Load the module like before
print Foo::fact(4),"\n"; # Call a function in package FooBar
</pre></div>
-->
<H2><a name="Perl5_nn25"></a>31.5 Input and output parameters</H2>
<p>
A common problem in some C programs is handling parameters passed as simple pointers. For
example:
</p>
<div class="code">
<pre>
void add(int x, int y, int *result) {
*result = x + y;
}
</pre>
</div>
<p>
or perhaps
</p>
<div class="code">
<pre>
int sub(int *x, int *y) {
return *x+*y;
}
</pre>
</div>
<p>
The easiest way to handle these situations is to use the <tt>typemaps.i</tt> file. For example:
</p>
<div class="code">
<pre>
%module example
%include "typemaps.i"
void add(int, int, int *OUTPUT);
int sub(int *INPUT, int *INPUT);
</pre>
</div>
<p>
In Perl, this allows you to pass simple values. For example:
</p>
<div class="targetlang">
<pre>
$a = example::add(3,4);
print "$a\n";
7
$b = example::sub(7,4);
print "$b\n";
3
</pre>
</div>
<p>
Notice how the <tt>INPUT</tt> parameters allow integer values to be passed instead of pointers
and how the <tt>OUTPUT</tt> parameter creates a return result.
</p>
<p>
If you don't want to use the names <tt>INPUT</tt> or <tt>OUTPUT</tt>, use the <tt>%apply</tt>
directive. For example:
</p>
<div class="code">
<pre>
%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);
</pre>
</div>
<p>
If a function mutates one of its parameters like this,
</p>
<div class="code">
<pre>
void negate(int *x) {
*x = -(*x);
}
</pre>
</div>
<p>
you can use <tt>INOUT</tt> like this:
</p>
<div class="code">
<pre>
%include "typemaps.i"
...
void negate(int *INOUT);
</pre>
</div>
<p>
In Perl, a mutated parameter shows up as a return value. For example:
</p>
<div class="targetlang">
<pre>
$a = example::negate(3);
print "$a\n";
-3
</pre>
</div>
<p>
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:
</p>
<div class="code">
<pre>
/* send message, return number of bytes sent, along with success code */
int send_message(char *text, int len, int *success);
</pre>
</div>
<p>
To wrap such a function, simply use the <tt>OUTPUT</tt> rule above. For example:
</p>
<div class="code">
<pre>
%module example
%include "typemaps.i"
%apply int *OUTPUT { int *success };
...
int send_message(char *text, int *success);
</pre>
</div>
<p>
When used in Perl, the function will return multiple values.
</p>
<div class="targetlang">
<pre>
($bytes, $success) = example::send_message("Hello World");
</pre>
</div>
<p>
Another common use of multiple return values are in query functions. For example:
</p>
<div class="code">
<pre>
void get_dimensions(Matrix *m, int *rows, int *columns);
</pre>
</div>
<p>
To wrap this, you might use the following:
</p>
<div class="code">
<pre>
%module example
%include "typemaps.i"
%apply int *OUTPUT { int *rows, int *columns };
...
void get_dimensions(Matrix *m, int *rows, *columns);
</pre>
</div>
<p>
Now, in Perl:
</p>
<div class="targetlang">
<pre>
($r,$c) = example::get_dimensions($m);
</pre>
</div>
<p>
In certain cases, it is possible to treat Perl references as C pointers. To do this, use the <tt>REFERENCE</tt> typemap. For
example:
</p>
<div class="code">
<pre>
%module example
%include "typemaps.i"
void add(int x, int y, int *REFERENCE);
</pre>
</div>
<p>
In Perl:
</p>
<div class="targetlang">
<pre>
use example;
$c = 0.0;
example::add(3,4,\$c);
print "$c\n";
7
</pre>
</div>
<p>
<b>Note:</b> The <tt>REFERENCE</tt> feature is only currently supported for numeric types (integers and floating point).
</p>
<H2><a name="Perl5_nn26"></a>31.6 Exception handling</H2>
<p>
The SWIG <tt>%exception</tt> directive can be used to create a
user-definable exception handler for converting exceptions in your
C/C++ program into Perl exceptions. The chapter on customization features
contains more details, but suppose you have a C++ class like the
following:
</p>
<div class="code"><pre>
class RangeError {}; // Used for an exception
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 item from the array and perform bounds checking.
double getitem(int i) {
if ((i &gt;= 0) &amp;&amp; (i &lt; n))
return ptr[i];
else
throw RangeError();
}
// Set an item in the array and perform bounds checking.
void setitem(int i, double val) {
if ((i &gt;= 0) &amp;&amp; (i &lt; n))
ptr[i] = val;
else {
throw RangeError();
}
}
};
</pre></div>
<p>
Since several methods in this class can throw an exception
for an out-of-bounds access, you might want to catch
this in the Perl extension by writing the following in an
interface file:
</p>
<div class="code"><pre>
%exception {
try {
$action
}
catch (RangeError) {
croak("Array index out-of-bounds");
}
}
class DoubleArray {
...
};
</pre></div>
<p>
The exception handling code is inserted directly into generated wrapper
functions. The <tt>$action</tt> variable is replaced with the C/C++
code being executed by the wrapper. When an exception handler
is defined, errors can be caught and used to gracefully generate a Perl error
instead of forcing the entire program to terminate with an uncaught error.
</p>
<p>
As shown, the exception handling code will be added to every wrapper function.
Since this is somewhat inefficient. You might consider refining the
exception handler to only apply to specific methods like this:
</p>
<div class="code">
<pre>
%exception getitem {
try {
$action
}
catch (RangeError) {
croak("Array index out-of-bounds");
}
}
%exception setitem {
try {
$action
}
catch (RangeError) {
croak("Array index out-of-bounds");
}
}
</pre>
</div>
<p>
In this case, the exception handler is only attached to methods and functions
named <tt>getitem</tt> and <tt>setitem</tt>.
</p>
<p>
If you had a lot of different methods, you can avoid extra typing by using a macro.
For example:
</p>
<div class="code">
<pre>
%define RANGE_ERROR
{
try {
$action
}
catch (RangeError) {
croak("Array index out-of-bounds");
}
}
%enddef
%exception getitem RANGE_ERROR;
%exception setitem RANGE_ERROR;
</pre>
</div>
<p>
Since SWIG's exception handling is user-definable, you are not limited to C++ exception handling.
See the chapter on "<a href="Customization.html#Customization">Customization features</a>" for more examples.
</p>
<p>
<b>Compatibility note:</b> In SWIG1.1, exceptions were defined using the older <tt>%except</tt> directive:
</p>
<div class="code">
<pre>
%except(python) {
try {
$function
}
catch (RangeError) {
croak("Array index out-of-bounds");
}
}
</pre>
</div>
<p>
This is still supported, but it is deprecated. The newer <tt>%exception</tt> directive provides the same
functionality, but it has additional capabilities that make it more powerful.
</p>
<H2><a name="Perl5_nn27"></a>31.7 Remapping datatypes with typemaps</H2>
<p>
This section describes how you can modify SWIG's default wrapping behavior
for various C/C++ datatypes using the <tt>%typemap</tt> directive. This
is an advanced topic that assumes familiarity with the Perl C API as well
as the material in the "<a href="Typemaps.html#Typemaps">Typemaps</a>" chapter.
</p>
<p>
Before proceeding, it should be stressed that typemaps are <em>not</em> 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-Perl interface.
</p>
<H3><a name="Perl5_nn28"></a>31.7.1 A simple typemap example</H3>
<p>
A typemap is nothing more than a code generation rule that is attached to
a specific C datatype. For example, to convert integers from Perl to C,
you might define a typemap like this:
</p>
<div class="code"><pre>
%module example
%typemap(in) int {
$1 = (int) SvIV($input);
printf("Received an integer : %d\n", $1);
}
...
%inline %{
extern int fact(int n);
%}
</pre></div>
<p>
Typemaps are always associated with some specific aspect of code generation.
In this case, the "in" method refers to the conversion of input arguments
to C/C++. The datatype <tt>int</tt> is the datatype to which the typemap
will be applied. The supplied C code is used to convert values. In this
code a number of special variable prefaced by a <tt>$</tt> are used. The
<tt>$1</tt> variable is placeholder for a local variable of type <tt>int</tt>.
The <tt>$input</tt> variable is the input object (usually a <tt>SV *</tt>).
</p>
<p>
When this example is used in Perl5, it will operate as follows:
</p>
<div class="targetlang"><pre>
use example;
$n = example::fact(6);
print "$n\n";
...
Output:
Received an integer : 6
720
</pre></div>
<p>
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 <tt>int</tt>, that typemap
applies to <tt>int</tt> and qualified variations such as <tt>const int</tt>. In addition,
the typemap system follows <tt>typedef</tt> declarations. For example:
</p>
<div class="targetlang">
<pre>
%typemap(in) int n {
$1 = (int) SvIV($input);
printf("n = %d\n",$1);
}
%inline %{
typedef int Integer;
extern int fact(Integer n); // Above typemap is applied
%}
</pre>
</div>
<p>
It should be noted that the matching of <tt>typedef</tt> only occurs in one direction. If you
defined a typemap for <tt>Integer</tt>, it is not applied to arguments of
type <tt>int</tt>.
</p>
<p>
Typemaps can also be defined for groups of consecutive arguments. For example:
</p>
<div class="targetlang">
<pre>
%typemap(in) (char *str, unsigned len) {
$1 = SvPV($input,$2);
};
int count(char c, char *str, unsigned len);
</pre>
</div>
<p>
When a multi-argument typemap is defined, the arguments are always handled as a single
Perl object. This allows the function to be used like this (notice how the length
parameter is omitted):
</p>
<div class="targetlang">
<pre>
example::count("e","Hello World");
1
&gt;&gt;&gt;
</pre>
</div>
<H3><a name="Perl5_nn29"></a>31.7.2 Perl5 typemaps</H3>
<p>
The previous section illustrated an "in" typemap for converting Perl objects to C.
A variety of different typemap methods are defined by the Perl module. For example,
to convert a C integer back into a Perl object, you might define an "out" typemap
like this:
</p>
<div class="targetlang">
<pre>
%typemap(out) int {
$result = sv_newmortal();
set_setiv($result, (IV) $1);
argvi++;
}
</pre>
</div>
<p>
The following typemap methods are available:
</p>
<p>
<tt>%typemap(in)</tt>
</p>
<div class="indent">
Converts Perl5 object to input function arguments.
</div>
<p>
<tt>%typemap(out)</tt>
</p>
<div class="indent">
Converts function return value to a Perl5 value.
</div>
<p>
<tt>%typemap(varin)</tt>
</p>
<div class="indent">
Converts a Perl5 object to a global variable.
</div>
<p>
<tt>%typemap(varout)</tt>
</p>
<div class="indent">
Converts a global variable to a Perl5 object.
</div>
<p>
<tt>%typemap(freearg)</tt>
</p>
<div class="indent">
Cleans up a function argument after a function call
</div>
<p>
<tt>%typemap(argout)</tt>
</p>
<div class="indent">
Output argument handling
</div>
<p>
<tt>%typemap(ret)</tt>
</p>
<div class="indent">
Clean up return value from a function.
</div>
<p>
<tt>%typemap(memberin)</tt>
</p>
<div class="indent">
Setting of C++ member data (all languages).
</div>
<p>
<tt>%typemap(memberout)</tt>
</p>
<div class="indent">
Return of C++ member data (all languages).
</div>
<p>
<tt>%typemap(check)</tt>
</p>
<div class="indent">
Check value of input parameter.
</div>
<H3><a name="Perl5_nn30"></a>31.7.3 Typemap variables</H3>
<p>
Within typemap code, a number of special variables prefaced with a <tt>$</tt> may appear.
A full list of variables can be found in the "<a href="Typemaps.html#Typemaps">Typemaps</a>" chapter.
This is a list of the most common variables:
</p>
<p>
<tt>$1</tt>
</p>
<div class="indent">
A C local variable corresponding to the actual type specified in the
<tt>%typemap</tt> directive. For input values, this is a C local variable
that's supposed to hold an argument value. For output values, this is
the raw result that's supposed to be returned to Perl.
</div>
<p>
<tt>$input</tt>
</p>
<div class="indent">
A Perl object holding the value of an argument of variable value.
</div>
<p>
<tt>$result</tt>
</p>
<div class="indent">
A Perl object that holds the result to be returned to Perl.
</div>
<p>
<tt>$1_name</tt>
</p>
<div class="indent">
The parameter name that was matched.
</div>
<p>
<tt>$1_type</tt>
</p>
<div class="indent">
The actual C datatype matched by the typemap.
</div>
<p>
<tt>$1_ltype</tt>
</p>
<div class="indent">
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 <tt>$1_type</tt>.
All arguments and local variables in wrapper functions are declared using this type so that their values can be
properly assigned.
</div>
<p>
<tt>$symname</tt>
</p>
<div class="indent">
The Perl name of the wrapper function being created.
</div>
<H3><a name="Perl5_nn31"></a>31.7.4 Useful functions</H3>
<p>
When writing typemaps, it is necessary to work directly with Perl5
objects. This, unfortunately, can be a daunting task. Consult the
"perlguts" man-page for all of the really ugly details. A short
summary of commonly used functions is provided here for reference. It
should be stressed that SWIG can be used quite effectively without
knowing any of these details--especially now that there are typemap
libraries that can already been written.
</p>
<p>
<b>Perl Integer Functions</b>
</p>
<div class="code">
<pre>
int SvIV(SV *);
void sv_setiv(SV *sv, IV value);
SV *newSViv(IV value);
int SvIOK(SV *);
</pre>
</div>
<p>
<b>Perl Floating Point Functions</b>
</p>
<div class="code">
<pre>
double SvNV(SV *);
void sv_setnv(SV *, double value);
SV *newSVnv(double value);
int SvNOK(SV *);
</pre>
</div>
<p>
<b>Perl String Functions</b>
</p>
<div class="code">
<pre>
char *SvPV(SV *, STRLEN len);
void sv_setpv(SV *, char *val);
void sv_setpvn(SV *, char *val, STRLEN len);
SV *newSVpv(char *value, STRLEN len);
int SvPOK(SV *);
void sv_catpv(SV *, char *);
void sv_catpvn(SV *, char *, STRLEN);
</pre>
</div>
<p>
<b>Perl References</b>
</p>
<div class="code">
<pre>
void sv_setref_pv(SV *, char *, void *ptr);
int sv_isobject(SV *);
SV *SvRV(SV *);
int sv_isa(SV *, char *0;
</pre>
</div>
<H2><a name="Perl5_nn32"></a>31.8 Typemap Examples</H2>
<p>
This section includes a few examples of typemaps. For more examples, you
might look at the files "<tt>perl5.swg</tt>" and "<tt>typemaps.i</tt>" in
the SWIG library.
</p>
<H3><a name="Perl5_nn33"></a>31.8.1 Converting a Perl5 array to a char **</H3>
<p>
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 Perl5 array
reference to be used as a char ** datatype.
</p>
<div class="code"><pre>
%module argv
// This tells SWIG to treat char ** as a special case
%typemap(in) char ** {
AV *tempav;
I32 len;
int i;
SV **tv;
if (!SvROK($input))
croak("Argument $argnum is not a reference.");
if (SvTYPE(SvRV($input)) != SVt_PVAV)
croak("Argument $argnum is not an array.");
tempav = (AV*)SvRV($input);
len = av_len(tempav);
$1 = (char **) malloc((len+2)*sizeof(char *));
for (i = 0; i &lt;= len; i++) {
tv = av_fetch(tempav, i, 0);
$1[i] = (char *) SvPV(*tv,PL_na);
}
$1[i] = NULL;
};
// This cleans up the char ** array after the function call
%typemap(freearg) char ** {
free($1);
}
// Creates a new Perl array and places a NULL-terminated char ** into it
%typemap(out) char ** {
AV *myav;
SV **svs;
int i = 0,len = 0;
/* Figure out how many elements we have */
while ($1[len])
len++;
svs = (SV **) malloc(len*sizeof(SV *));
for (i = 0; i &lt; len ; i++) {
svs[i] = sv_newmortal();
sv_setpv((SV*)svs[i],$1[i]);
};
myav = av_make(len,svs);
free(svs);
$result = newRV_noinc((SV*)myav);
sv_2mortal($result);
argvi++;
}
// Now a few test functions
%inline %{
int print_args(char **argv) {
int i = 0;
while (argv[i]) {
printf("argv[%d] = %s\n", i,argv[i]);
i++;
}
return i;
}
// Returns a char ** list
char **get_args() {
static char *values[] = { "Dave", "Mike", "Susan", "John", "Michelle", 0};
return &amp;values[0];
}
%}
</pre></div>
<p>
When this module is compiled, the wrapped C functions can be used in a
Perl script as follows:
</p>
<div class="targetlang"><pre>
use argv;
@a = ("Dave", "Mike", "John", "Mary"); # Create an array of strings
argv::print_args(\@a); # Pass it to our C function
$b = argv::get_args(); # Get array of strings from C
print @$b,"\n"; # Print it out
</pre></div>
<H3><a name="Perl5_nn34"></a>31.8.2 Return values</H3>
<p>
Return values are placed on the argument stack of each wrapper
function. The current value of the argument stack pointer is
contained in a variable <tt>argvi</tt>. Whenever a new output value
is added, it is critical that this value be incremented. For multiple
output values, the final value of <tt>argvi</tt> should be the total
number of output values.
</p>
<p>
The total number of return values should not exceed the number of
input values unless you explicitly extend the argument stack. This
can be done using the <tt>EXTEND()</tt> macro as in:
</p>
<div class="code"><pre>
%typemap(argout) int *OUTPUT {
if (argvi &gt;= items) {
EXTEND(sp,1); /* Extend the stack by 1 object */
}
$result = sv_newmortal();
sv_setiv($target,(IV) *($1));
argvi++;
}
</pre></div>
<H3><a name="Perl5_nn35"></a>31.8.3 Returning values from arguments</H3>
<p>
Sometimes it is desirable for a function to return a value in one of
its arguments. This example describes the implementation of the <tt>OUTPUT</tt> typemap.
</p>
<div class="code"><pre>
%module return
// This tells SWIG to treat an double * argument with name 'OutDouble' as
// an output value.
%typemap(argout) double *OUTPUT {
$result = sv_newmortal();
sv_setnv($result, *$input);
argvi++; /* Increment return count -- important! */
}
// We don't care what the input value is. Ignore, but set to a temporary variable
%typemap(in,numinputs=0) double *OUTPUT(double junk) {
$1 = &amp;junk;
}
// Now a function to test it
%{
/* Returns the first two input arguments */
int multout(double a, double b, double *out1, double *out2) {
*out1 = a;
*out2 = b;
return 0;
};
%}
// If we name both parameters OutDouble both will be output
int multout(double a, double b, double *OUTPUT, double *OUTPUT);
...
</pre></div>
<p>
When this function is called, the output arguments are appended to the stack used
to return results. This shows up an array in Perl.
For example:
</p>
<div class="targetlang"><pre>
@r = multout(7,13);
print "multout(7,13) = @r\n";
($x,$y) = multout(7,13);
</pre></div>
<H3><a name="Perl5_nn36"></a>31.8.4 Accessing array structure members</H3>
<p>
Consider the following data structure:
</p>
<div class="code"><pre>
#define SIZE 8
typedef struct {
int values[SIZE];
...
} Foo;
</pre></div>
<p>
By default, SWIG doesn't know how to the handle the values structure
member it's an array, not a pointer. In this case, SWIG makes the array member
read-only. Reading will simply return a pointer to the first item in the array.
To make the member writable, a "memberin" typemap can be used.
</p>
<div class="code"><pre>
%typemap(memberin) int [SIZE] {
int i;
for (i = 0; i &lt; SIZE; i++) {
$1[i] = $input[i];
}
}
</pre></div>
<p>
Whenever a <tt>int [SIZE]</tt> member is encountered in a structure
or class, this typemap provides a safe mechanism for setting its
value.
</p>
<p>
As in the previous example, the typemap can be generalized for any dimension.
For example:
</p>
<div class="code"><pre>
%typemap(memberin) int [ANY] {
int i;
for (i = 0; i &lt; $1_dim0; i++) {
$1[i] = $input[i];
}
}
</pre></div>
<p>
When setting structure members, the input object is always assumed to
be a C array of values that have already been converted from the
target language. Because of this, the <tt>memberin</tt> typemap is
almost always combined with the use of an "in" typemap. For example,
the "in" typemap in the previous section would be used to convert an
<tt>int[]</tt> array to C whereas the "memberin" typemap would be used
to copy the converted array into a C data structure.
</p>
<H3><a name="Perl5_nn37"></a>31.8.5 Turning Perl references into C pointers</H3>
<p>
A frequent confusion on the SWIG mailing list is errors caused by the
mixing of Perl references and C pointers. For example, suppose you
have a C function that modifies its arguments like this:
</p>
<div class="code"><pre>
void add(double a, double b, double *c) {
*c = a + b;
}
</pre></div>
<p>
A common misinterpretation of this function is the following Perl script:
</p>
<div class="targetlang"><pre>
# Perl script
$a = 3.5;
$b = 7.5;
$c = 0.0; # Output value
add($a,$b,\$c); # Place result in c (Except that it doesn't work)
</pre></div>
<p>
To make this work with a reference, you can use a typemap such as this:
</p>
<div class="code"><pre>
%typemap(in) double * (double dvalue) {
SV* tempsv;
if (!SvROK($input)) {
croak("expected a reference\n");
}
tempsv = SvRV($input);
if ((!SvNOK(tempsv)) &amp;&amp; (!SvIOK(tempsv))) {
croak("expected a double reference\n");
}
dvalue = SvNV(tempsv);
$1 = &amp;dvalue;
}
%typemap(argout) double * {
SV *tempsv;
tempsv = SvRV($input);
sv_setnv(tempsv, *$1);
}
</pre></div>
<p>
Now, if you place this before the add function, you can do this:
</p>
<div class="targetlang"><pre>
$a = 3.5;
$b = 7.5;
$c = 0.0;
add($a,$b,\$c); # Now it works!
print "$c\n";
</pre></div>
<H3><a name="Perl5_nn38"></a>31.8.6 Pointer handling</H3>
<p>
Occasionally, it might be necessary to convert pointer values that have
been stored using the SWIG typed-pointer representation. To convert a pointer from Perl to C, the following
function is used:
</p>
<p>
<tt>
int SWIG_ConvertPtr(SV *obj, void **ptr, swig_type_info *ty, int flags)
</tt>
</p>
<div class="indent">
Converts a Perl object <tt>obj</tt> to a C pointer. The result of the conversion is placed
into the pointer located at <tt>ptr</tt>. <tt>ty</tt> is a SWIG type descriptor structure.
<tt>flags</tt> is used to handle error checking and other aspects of conversion. <tt>flags</tt> is
currently undefined and reserved for future expansion. Returns 0 on success and -1 on error.
</div>
<p>
<tt>
void *SWIG_MakePtr(SV *obj, void *ptr, swig_type_info *ty, int flags)</tt>
</p>
<div class="indent">
Creates a new Perl pointer object. <tt>obj</tt> is a Perl SV that has been initialized to hold the result,
<tt>ptr</tt> is the pointer to convert, <tt>ty</tt> is the SWIG type descriptor structure that
describes the type, and <tt>flags</tt> is a flag that controls properties of the conversion. <tt>flags</tt> is currently undefined
and reserved.
</div>
<p>
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 <tt>Foo *</tt>, the type descriptor structure
is usually accessed as follows:
</p>
<div class="code">
<pre>
Foo *f;
if (SWIG_ConvertPtr($input, (void **) &amp;f, SWIGTYPE_p_Foo, 0) == -1) return NULL;
SV *sv = sv_newmortal();
SWIG_MakePtr(sv, f, SWIGTYPE_p_Foo, 0);
</pre>
</div>
<p>
In a typemap, the type descriptor should always be accessed using the special typemap
variable <tt>$1_descriptor</tt>. For example:
</p>
<div class="code">
<pre>
%typemap(in) Foo * {
if ((SWIG_ConvertPtr($input,(void **) &amp;$1, $1_descriptor,0)) == -1) return NULL;
}
</pre>
</div>
<p>
If necessary, the descriptor for any type can be obtained using the <tt>$descriptor()</tt> macro in a typemap.
For example:
</p>
<div class="code">
<pre>
%typemap(in) Foo * {
if ((SWIG_ConvertPtr($input,(void **) &amp;$1, $descriptor(Foo *), 0)) == -1) return NULL;
}
</pre>
</div>
<H2><a name="Perl5_nn39"></a>31.9 Proxy classes</H2>
<p>
<b>Out of date. Needs update.</b>
</p>
<p>
Using the low-level procedural interface, SWIG can also construct a
high-level object oriented interface to C structures and C++ classes.
This is done by constructing a Perl proxy class (also known as a shadow class)
that provides an OO wrapper
to the underlying code. This section describes the implementation
details of the proxy interface.
</p>
<H3><a name="Perl5_nn40"></a>31.9.1 Preliminaries</H3>
<p>
Proxy classes, are generated by default. If you want to turn them off, use the <tt>-noproxy</tt> command line option.
For example:
</p>
<div class="code">
<pre>
$ swig -c++ -perl -noproxy example.i
</pre>
</div>
<p>
When proxy classes are used, SWIG moves all of the low-level procedural wrappers to
another package name. By default, this package is named 'modulec' where 'module' is the name of the module
you provided with the <tt>%module</tt> directive. Then, in place of the original module,
SWIG creates a collection of high-level Perl wrappers. In your scripts, you will use these
high level wrappers. The wrappers, in turn, interact with the low-level procedural module.
</p>
<H3><a name="Perl5_nn41"></a>31.9.2 Structure and class wrappers</H3>
<p>
Suppose you have the following SWIG interface file:
</p>
<div class="code"><pre>
%module example
struct Vector {
Vector(double x, double y, double z);
~Vector();
double x,y,z;
};
</pre></div>
<p>
When wrapped, SWIG creates the following set of low-level accessor
functions as described in previous sections.
</p>
<div class="code"><pre>
Vector *new_Vector(double x, double y, double z);
void delete_Vector(Vector *v);
double Vector_x_get(Vector *v);
double Vector_x_set(Vector *v, double value);
double Vector_y_get(Vector *v);
double Vector_y_set(Vector *v, double value);
double Vector_z_get(Vector *v);
double Vector_z_set(Vector *v, double value);
</pre></div>
<p>
However, when proxy classes are enabled, these accessor functions are
wrapped inside a Perl class like this:
</p>
<div class="targetlang"><pre>
package example::Vector;
@ISA = qw( example );
%OWNER = ();
%BLESSEDMEMBERS = ();
sub new () {
my $self = shift;
my @args = @_;
$self = vectorc::new_Vector(@args);
return undef if (!defined($self));
bless $self, "example::Vector";
$OWNER{$self} = 1;
my %retval;
tie %retval, "example::Vector", $self;
return bless \%retval,"Vector";
}
sub DESTROY {
return unless $_[0]-&gt;isa('HASH');
my $self = tied(%{$_[0]});
delete $ITERATORS{$self};
if (exists $OWNER{$self}) {
examplec::delete_Vector($self));
delete $OWNER{$self};
}
sub FETCH {
my ($self,$field) = @_;
my $member_func = "vectorc::Vector_${field}_get";
my $val = &amp;$member_func($self);
if (exists $BLESSEDMEMBERS{$field}) {
return undef if (!defined($val));
my %retval;
tie %retval,$BLESSEDMEMBERS{$field},$val;
return bless \%retval, $BLESSEDMEMBERS{$field};
}
return $val;
}
sub STORE {
my ($self,$field,$newval) = @_;
my $member_func = "vectorc::Vector_${field}_set";
if (exists $BLESSEDMEMBERS{$field}) {
&amp;$member_func($self,tied(%{$newval}));
} else {
&amp;$member_func($self,$newval);
}
}
</pre></div>
<p>
Each structure or class is mapped into a Perl package of the same
name. The C++ constructors and destructors are mapped into
constructors and destructors for the package and are always named
"new" and "DESTROY". The constructor always returns a tied hash
table. This hash table is used to access the member variables of a
structure in addition to being able to invoke member functions. The
<tt>%OWNER</tt> and <tt>%BLESSEDMEMBERS</tt> hash tables are used
internally and described shortly.
</p>
<p>
To use our new proxy class we can simply do the following:
</p>
<div class="targetlang"><pre>
# Perl code using Vector class
$v = new Vector(2,3,4);
$w = Vector-&gt;new(-1,-2,-3);
# Assignment of a single member
$v-&gt;{x} = 7.5;
# Assignment of all members
%$v = ( x=&gt;3,
y=&gt;9,
z=&gt;-2);
# Reading members
$x = $v-&gt;{x};
# Destruction
$v-&gt;DESTROY();
</pre></div>
<H3><a name="Perl5_nn42"></a>31.9.3 Object Ownership</H3>
<p>
In order for proxy classes to work properly, it is necessary for Perl
to manage some mechanism of object ownership. Here's the crux of the
problem---suppose you had a function like this:
</p>
<div class="code"><pre>
Vector *Vector_get(Vector *v, int index) {
return &amp;v[i];
}
</pre></div>
<p>
This function takes a Vector pointer and returns a pointer to another
Vector. Such a function might be used to manage arrays or lists of
vectors (in C). Now contrast this function with the constructor for a
Vector object:
</p>
<div class="code"><pre>
Vector *new_Vector(double x, double y, double z) {
Vector *v;
v = new Vector(x,y,z); // Call C++ constructor
return v;
}
</pre></div>
<p>
Both functions return a Vector, but the constructor is returning a
brand-new Vector while the other function is returning a Vector that
was already created (hopefully). In Perl, both vectors will be
indistinguishable---clearly a problem considering that we would
probably like the newly created Vector to be destroyed when we are
done with it.
</p>
<p>
To manage these problems, each class contains two methods that access
an internal hash table called <tt>%OWNER</tt>. This hash keeps a list
of all of the objects that Perl knows that it has created. This
happens in two cases: (1) when the constructor has been called, and
(2) when a function implicitly creates a new object (as is done when
SWIG needs to return a complex datatype by value). When the
destructor is invoked, the Perl proxy class module checks the
<tt>%OWNER</tt> hash to see if Perl created the object. If so, the
C/C++ destructor is invoked. If not, we simply destroy the Perl
object and leave the underlying C object alone (under the assumption
that someone else must have created it).
</p>
<p>
This scheme works remarkably well in practice but it isn't foolproof.
In fact, it will fail if you create a new C object in Perl, pass it on
to a C function that remembers the object, and then destroy the
corresponding Perl object (this situation turns out to come up
frequently when constructing objects like linked lists and trees).
When C takes possession of an object, you can change Perl's ownership
by simply deleting the object from the <tt>%OWNER</tt> hash. This is
done using the <tt>DISOWN</tt> method.
</p>
<div class="targetlang"><pre>
# Perl code to change ownership of an object
$v = new Vector(x,y,z);
$v-&gt;DISOWN();
</pre></div>
<p>
To acquire ownership of an object, the <tt>ACQUIRE</tt> method can be used.
</p>
<div class="targetlang"><pre>
# Given Perl ownership of a file
$u = Vector_get($v);
$u-&gt;ACQUIRE();
</pre></div>
<p>
As always, a little care is in order. SWIG does not provide reference
counting, garbage collection, or advanced features one might find in
sophisticated languages.
</p>
<H3><a name="Perl5_nn43"></a>31.9.4 Nested Objects</H3>
<p>
Suppose that we have a new object that looks like this:
</p>
<div class="code"><pre>
struct Particle {
Vector r;
Vector v;
Vector f;
int type;
}
</pre></div>
<p>
In this case, the members of the structure are complex objects that
have already been encapsulated in a Perl proxy class. To handle
these correctly, we use the <tt>%BLESSEDMEMBERS</tt> hash which would
look like this (along with some supporting code):
</p>
<div class="targetlang"><pre>
package Particle;
...
%BLESSEDMEMBERS = (
r =&gt; `Vector',
v =&gt; `Vector',
f =&gt; `Vector',
);
</pre></div>
<p>
When fetching members from the structure, <tt>%BLESSEDMEMBERS</tt> is
checked. If the requested field is present, we create a tied-hash
table and return it. If not, we just return the corresponding member
unmodified.
</p>
<p>
This implementation allows us to operate on nested structures as follows:
</p>
<div class="targetlang"><pre>
# Perl access of nested structure
$p = new Particle();
$p-&gt;{f}-&gt;{x} = 0.0;
%${$p-&gt;{v}} = ( x=&gt;0, y=&gt;0, z=&gt;0);
</pre></div>
<H3><a name="Perl5_nn44"></a>31.9.5 Proxy Functions</H3>
<p>
When functions take arguments involving a complex object, it is
sometimes necessary to write a proxy function. For example:
</p>
<div class="code"><pre>
double dot_product(Vector *v1, Vector *v2);
</pre></div>
<p>
Since Vector is an object already wrapped into a proxy class, we need
to modify this function to accept arguments that are given in the form
of tied hash tables. This is done by creating a Perl function like
this:
</p>
<div class="targetlang"><pre>
sub dot_product {
my @args = @_;
$args[0] = tied(%{$args[0]}); # Get the real pointer values
$args[1] = tied(%{$args[1]});
my $result = vectorc::dot_product(@args);
return $result;
}
</pre></div>
<p>
This function replaces the original function, but operates in an
identical manner.
</p>
<H3><a name="Perl5_nn45"></a>31.9.6 Inheritance</H3>
<p>
Simple C++ inheritance is handled using the Perl <tt>@ISA</tt> array
in each class package. For example, if you have the following
interface file:
</p>
<div class="code"><pre>
// shapes.i
// SWIG interface file for shapes class
%module shapes
%{
#include "shapes.h"
%}
class Shape {
public:
virtual double area() = 0;
virtual double perimeter() = 0;
void set_location(double x, double y);
};
class Circle : public Shape {
public:
Circle(double radius);
~Circle();
double area();
double perimeter();
};
class Square : public Shape {
public:
Square(double size);
~Square();
double area();
double perimeter();
}
</pre></div>
<p>
The resulting, Perl wrapper class will create the following code:
</p>
<div class="targetlang"><pre>
Package Shape;
@ISA = (shapes);
...
Package Circle;
@ISA = (shapes Shape);
...
Package Square;
@ISA = (shapes Shape);
</pre></div>
<p>
The <tt>@ISA</tt> array determines where to look for methods of a
particular class. In this case, both the <tt>Circle</tt> and
<tt>Square</tt> classes inherit functions from <tt>Shape</tt> so we'll
want to look in the <tt>Shape</tt> base class for them. All classes
also inherit from the top-level module <tt>shapes</tt>. This is
because certain common operations needed to implement proxy classes
are implemented only once and reused in the wrapper code for various
classes and structures.
</p>
<p>
Since SWIG proxy classes are implemented in Perl, it is easy to
subclass from any SWIG generated class. To do this, simply put the
name of a SWIG class in the <tt>@ISA</tt> array for your new
class. However, be forewarned that this is not a trivial problem. In
particular, inheritance of data members is extremely tricky (and I'm
not even sure if it really works).
</p>
<H3><a name="Perl5_nn46"></a>31.9.7 Modifying the proxy methods</H3>
<p>
It is possible to override the SWIG generated proxy/shadow methods, using <tt>%feature("shadow")</tt>.
It works like all the other <a href="Customization.html#Customization_features">%feature directives</a>.
Here is a simple example showing how to add some Perl debug code to the constructor:
</p>
<div class="targetlang"><pre>
/* Let's make the constructor of the class Square more verbose */
%feature("shadow") Square(double w)
%{
sub new {
my $pkg = shift;
my $self = examplec::new_Square(@_);
print STDERR "Constructed an @{[ref($self)]}\n";
bless $self, $pkg if defined($self);
}
%}
class Square {
public:
Square(double w);
...
};
</pre></div>
<H2><a name="Perl5_nn47"></a>31.10 Adding additional Perl code</H2>
<p>
If writing support code in C isn't enough, it is also possible to write code in
Perl. This code gets inserted in to the <tt>.pm</tt> file created by SWIG. One
use of Perl code might be to supply a high-level interface to certain functions.
For example:
</p>
<div class="code">
<pre>
void set_transform(Image *im, double x[4][4]);
...
/* Rewrite the high level interface to set_transform */
%perlcode %{
sub set_transform
{
my ($im, $x) = @_;
my $a = new_mat44();
for (my $i = 0; $i &lt; 4, $i++)
{
for (my $j = 0; $j &lt; 4, $j++)
{
mat44_set($a, $i, $j, $x-&gt;[i][j])
}
}
example.set_transform($im, $a);
free_mat44($a);
}
%}
</pre>
</div>
<p>
In this example, <tt>set_transform()</tt> provides a high-level Perl interface built on top of
low-level helper functions. For example, this code now seems to work:
</p>
<div class="targetlang">
<pre>
my $a =
[[1,0,0,0],
[0,1,0,0],
[0,0,1,0],
[0,0,0,1]];
set_transform($im, $a);
</pre>
</div>
<H2><a name="Perl5_directors"></a>31.11 Cross language polymorphism</H2>
<p>
Proxy classes provide a more natural, object-oriented way to access
extension classes. As described above, each proxy instance has an
associated C++ instance, and method calls to the proxy are passed to the
C++ instance transparently via C wrapper functions.
</p>
<p>
This arrangement is asymmetric in the sense that no corresponding
mechanism exists to pass method calls down the inheritance chain from
C++ to Perl. In particular, if a C++ class has been extended in Perl
(by extending the proxy class), these extensions will not be visible
from C++ code. Virtual method calls from C++ are thus not able access
the lowest implementation in the inheritance chain.
</p>
<p>
Changes have been made to SWIG to address this problem and
make the relationship between C++ classes and proxy classes more
symmetric. To achieve this goal, new classes called directors are
introduced at the bottom of the C++ inheritance chain. The job of the
directors is to route method calls correctly, either to C++
implementations higher in the inheritance chain or to Perl
implementations lower in the inheritance chain. The upshot is that C++
classes can be extended in Perl and from C++ these extensions look
exactly like native C++ classes. Neither C++ code nor Perl code needs
to know where a particular method is implemented: the combination of
proxy classes, director classes, and C wrapper functions takes care of
all the cross-language method routing transparently.
</p>
<H3><a name="Perl5_nn48"></a>31.11.1 Enabling directors</H3>
<p>
The director feature is disabled by default. To use directors you
must make two changes to the interface file. First, add the "directors"
option to the %module directive, like this:
</p>
<div class="code">
<pre>
%module(directors="1") modulename
</pre>
</div>
<p>
Without this option no director code will be generated. Second, you
must use the %feature("director") directive to tell SWIG which classes
and methods should get directors. The %feature directive can be applied
globally, to specific classes, and to specific methods, like this:
</p>
<div class="code">
<pre>
// generate directors for all classes that have virtual methods
%feature("director");
// generate directors for all virtual methods in class Foo
%feature("director") Foo;
</pre>
</div>
<p>
You can use the %feature("nodirector") directive to turn off
directors for specific classes or methods. So for example,
</p>
<div class="code">
<pre>
%feature("director") Foo;
%feature("nodirector") Foo::bar;
</pre>
</div>
<p>
will generate directors for all virtual methods of class Foo except
bar().
</p>
<p>
Directors can also be generated implicitly through inheritance.
In the following, class Bar will get a director class that handles
the methods one() and two() (but not three()):
</p>
<div class="code">
<pre>
%feature("director") Foo;
class Foo {
public:
Foo(int foo);
virtual void one();
virtual void two();
};
class Bar: public Foo {
public:
virtual void three();
};
</pre>
</div>
<p>
then at the Perl side you can define
</p>
<div class="targetlang">
<pre>
use mymodule;
package MyFoo;
use base 'mymodule::Foo';
sub one {
print "one from Perl\n";
}
</pre>
</div>
<H3><a name="Perl5_nn49"></a>31.11.2 Director classes</H3>
<p>
For each class that has directors enabled, SWIG generates a new class
that derives from both the class in question and a special
<tt>Swig::Director</tt> class. These new classes, referred to as director
classes, can be loosely thought of as the C++ equivalent of the Perl
proxy classes. The director classes store a pointer to their underlying
Perl object and handle various issues related to object ownership.
</p>
<p>
For simplicity let's ignore the <tt>Swig::Director</tt> class and refer to the
original C++ class as the director's base class. By default, a director
class extends all virtual methods in the inheritance chain of its base
class (see the preceding section for how to modify this behavior).
Thus all virtual method calls, whether they originate in C++ or in
Perl via proxy classes, eventually end up in at the implementation in
the director class. The job of the director methods is to route these
method calls to the appropriate place in the inheritance chain. By
"appropriate place" we mean the method that would have been called if
the C++ base class and its extensions in Perl were seamlessly
integrated. That seamless integration is exactly what the director
classes provide, transparently skipping over all the messy extension API
glue that binds the two languages together.
</p>
<p>
In reality, the "appropriate place" is one of only two possibilities:
C++ or Perl. Once this decision is made, the rest is fairly easy. If
the correct implementation is in C++, then the lowest implementation of
the method in the C++ inheritance chain is called explicitly. If the
correct implementation is in Perl, the Perl API is used to call the
method of the underlying Perl object (after which the usual virtual
method resolution in Perl automatically finds the right
implementation).
</p>
<p>
Now how does the director decide which language should handle the method call?
The basic rule is to handle the method in Perl, unless there's a good
reason not to. The reason for this is simple: Perl has the most
"extended" implementation of the method. This assertion is guaranteed,
since at a minimum the Perl proxy class implements the method. If the
method in question has been extended by a class derived from the proxy
class, that extended implementation will execute exactly as it should.
If not, the proxy class will route the method call into a C wrapper
function, expecting that the method will be resolved in C++. The wrapper
will call the virtual method of the C++ instance, and since the director
extends this the call will end up right back in the director method. Now
comes the "good reason not to" part. If the director method were to blindly
call the Perl method again, it would get stuck in an infinite loop. We avoid this
situation by adding special code to the C wrapper function that tells
the director method to not do this. The C wrapper function compares the
pointer to the Perl object that called the wrapper function to the
pointer stored by the director. If these are the same, then the C
wrapper function tells the director to resolve the method by calling up
the C++ inheritance chain, preventing an infinite loop.
</p>
<p>
One more point needs to be made about the relationship between director
classes and proxy classes. When a proxy class instance is created in
Perl, SWIG creates an instance of the original C++ class.
This is exactly what happens without directors and
is true even if directors are enabled for the particular class in
question. When a class <i>derived</i> from a proxy class is created,
however, SWIG then creates an instance of the corresponding C++ director
class. The reason for this difference is that user-defined subclasses
may override or extend methods of the original class, so the director
class is needed to route calls to these methods correctly. For
unmodified proxy classes, all methods are ultimately implemented in C++
so there is no need for the extra overhead involved with routing the
calls through Perl.
</p>
<H3><a name="Perl5_nn50"></a>31.11.3 Ownership and object destruction</H3>
<p>
Memory management issues are slightly more complicated with directors
than for proxy classes alone. Perl instances hold a pointer to the
associated C++ director object, and the director in turn holds a pointer
back to a Perl object. By default, proxy classes own their C++
director object and take care of deleting it when they are garbage
collected.
</p>
<p>
This relationship can be reversed by calling the special
<tt>DISOWN()</tt> method of the proxy class. After calling this
method the director
class increments the reference count of the Perl object. When the
director class is deleted it decrements the reference count. Assuming no
outstanding references to the Perl object remain, the Perl object
will be destroyed at the same time. This is a good thing, since
directors and proxies refer to each other and so must be created and
destroyed together. Destroying one without destroying the other will
likely cause your program to segfault.
</p>
<p>
Also note that due to the proxy implementation, the <tt>DESTROY()</tt>
method on directors can be called for several reasons, many of which
have little to do with the teardown of an object instance. To help
disambiguate this, a second argument is added to the <tt>DESTROY()</tt>
call when a C++ director object is being released. So, to avoid running
your clean-up code when an object is not really going away, or after it
has already been reclaimed, it is suggested that custom destructors in
Perl subclasses looks something like:
</p>
<div class="targetlang">
<pre>
sub DESTROY {
my($self, $final) = @_;
if($final) {
# real teardown code
}
shift-&gt;SUPER::DESTROY(@_);
}
</pre>
</div>
<H3><a name="Perl5_nn51"></a>31.11.4 Exception unrolling</H3>
<p>
With directors routing method calls to Perl, and proxies routing them
to C++, the handling of exceptions is an important concern. By default, the
directors ignore exceptions that occur during method calls that are
resolved in Perl. To handle such exceptions correctly, it is necessary
to temporarily translate them into C++ exceptions. This can be done with
the %feature("director:except") directive. The following code should
suffice in most cases:
</p>
<div class="code">
<pre>
%feature("director:except") {
if ($error != NULL) {
throw Swig::DirectorMethodException();
}
}
</pre>
</div>
<p>
This code will check the Perl error state after each method call from
a director into Perl, and throw a C++ exception if an error occurred.
This exception can be caught in C++ to implement an error handler.
</p>
<p>
It may be the case that a method call originates in Perl, travels up
to C++ through a proxy class, and then back into Perl via a director
method. If an exception occurs in Perl at this point, it would be nice
for that exception to find its way back to the original caller. This can
be done by combining a normal %exception directive with the
<tt>director:except</tt> handler shown above. Here is an example of a
suitable exception handler:
</p>
<div class="code">
<pre>
%exception {
try { $action }
catch (Swig::DirectorException &amp;e) { SWIG_fail; }
}
</pre>
</div>
<p>
The class Swig::DirectorException used in this example is actually a
base class of Swig::DirectorMethodException, so it will trap this
exception. Because the Perl error state is still set when
Swig::DirectorMethodException is thrown, Perl will register the
exception as soon as the C wrapper function returns.
</p>
<H3><a name="Perl5_nn52"></a>31.11.5 Overhead and code bloat</H3>
<p>
Enabling directors for a class will generate a new director method for
every virtual method in the class' inheritance chain. This alone can
generate a lot of code bloat for large hierarchies. Method arguments
that require complex conversions to and from target language types can
result in large director methods. For this reason it is recommended that
you selectively enable directors only for specific classes that are
likely to be extended in Perl and used in C++.
</p>
<p>
Compared to classes that do not use directors, the call routing in the
director methods does add some overhead. In particular, at least one
dynamic cast and one extra function call occurs per method call from
Perl. Relative to the speed of Perl execution this is probably
completely negligible. For worst case routing, a method call that
ultimately resolves in C++ may take one extra detour through Perl in
order to ensure that the method does not have an extended Perl
implementation. This could result in a noticeable overhead in some cases.
</p>
<p>
Although directors make it natural to mix native C++ objects with Perl
objects (as director objects) via a common base class pointer, one
should be aware of the obvious fact that method calls to Perl objects
will be much slower than calls to C++ objects. This situation can be
optimized by selectively enabling director methods (using the %feature
directive) for only those methods that are likely to be extended in
Perl.
</p>
<H3><a name="Perl5_nn53"></a>31.11.6 Typemaps</H3>
<p>
Typemaps for input and output of most of the basic types from director
classes have been written. These are roughly the reverse of the usual
input and output typemaps used by the wrapper code. The typemap
operation names are 'directorin', 'directorout', and 'directorargout'.
The director code does not currently use any of the other kinds of typemaps.
It is not clear at this point which kinds are appropriate and
need to be supported.
</p>
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