jyapayne/swig
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Merge https://swig.svn.sourceforge.net/svnroot/swig/branches/gsoc2009-vmiklos/

Browse source 
git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk@11434 626c5289-ae23-0410-ae9c-e8d60b6d4f22
This commit is contained in:
Miklos Vajna 2009-07-22 11:08:29 +00:00
commit 0249eea389
53 changed files with 2838 additions and 66 deletions
Download patch file Download diff file Expand all files Collapse all files

386
Doc/Manual/Php.html
View file

@ -32,6 +32,16 @@
</ul>
<li><a href="#Php_nn2_7">PHP Pragmas, Startup and Shutdown code</a>
</ul>
<li><a href="#Php_nn3">Cross language polymorphism</a>
<ul>
<li><a href="#Php_nn3_1">Enabling directors</a>
<li><a href="#Php_nn3_2">Director classes</a>
<li><a href="#Php_nn3_3">Ownership and object destruction</a>
<li><a href="#Php_nn3_4">Exception unrolling</a>
<li><a href="#Php_nn3_5">Overhead and code bloat</a>
<li><a href="#Php_nn3_6">Typemaps</a>
<li><a href="#Php_nn3_7">Miscellaneous</a>
</ul>
</ul>
</div>
<!-- INDEX -->
@ -866,5 +876,381 @@ The <tt>%rinit</tt> and <tt>%rshutdown</tt> statements insert code
into the request init and shutdown code respectively.
</p>
<H2><a name="Php_nn3"></a>29.3 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 PHP. In particular, if a C++ class has been extended in PHP
(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 1.3.18 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. Support for generating PHP classes
has been added in SWIG 1.3.40. The job of the directors is to route
method calls correctly, either to C++ implementations higher in the
inheritance chain or to PHP implementations lower in the inheritance
chain. The upshot is that C++ classes can be extended in PHP and from
C++ these extensions look exactly like native C++ classes. Neither C++
code nor PHP 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="Php_nn3_1"></a>29.3.1 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;
// generate a director for just Foo::bar()
%feature("director") Foo::bar;
</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 PHP side you can define
</p>
<div class="targetlang">
<pre>
require("mymodule.php");
class MyFoo extends Foo {
function one() {
print "one from php\n";
}
}
</pre>
</div>
<H3><a name="Php_nn3_2"></a>29.3.2 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 PHP
proxy classes. The director classes store a pointer to their underlying
PHP object. Indeed, this is quite similar to the "_cPtr" and "thisown"
members of the PHP proxy classes.
</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
PHP 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 PHP 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 PHP. 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 PHP, the Zend API is used to call the method of the
underlying PHP object (after which the usual virtual method resolution
in PHP 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 PHP, unless there's a good
reason not to. The reason for this is simple: PHP has the most
"extended" implementation of the method. This assertion is guaranteed,
since at a minimum the PHP 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 PHP 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
called and the declaring class name of the given method. If these are
not 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
PHP, SWIG creates an instance of the original C++ class and assigns it
to <tt>-&gt;_cPtr</tt>. 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 PHP.
</p>
<H3><a name="Php_nn3_3"></a>29.3.3 Ownership and object destruction</H3>
<p>
Memory management issues are slightly more complicated with directors
than for proxy classes alone. PHP instances hold a pointer to the
associated C++ director object, and the director in turn holds a pointer
back to the PHP 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>-&gt;thisown</tt> property of the proxy class. After setting this
property to <tt>0</tt>, the director class no longer destroys the PHP
object. Assuming no outstanding references to the PHP object remain,
the PHP 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>
Here is an example:
</p>
<div class="code">
<pre>
class Foo {
public:
...
};
class FooContainer {
public:
void addFoo(Foo *);
...
};
</pre>
</div>
<br>
<div class="targetlang">
<pre>
$c = new FooContainer();
$a = new Foo();
$a-&gt;thisown = 0;
$c-&gt;addFoo($a);
</pre>
</div>
<p>
In this example, we are assuming that FooContainer will take care of
deleting all the Foo pointers it contains at some point.
</p>
<H3><a name="Php_nn3_4"></a>29.3.4 Exception unrolling</H3>
<p>
With directors routing method calls to PHP, 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 PHP. 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 == FAILURE) {
throw Swig::DirectorMethodException();
}
}
</pre>
</div>
<p>
This code will check the PHP error state after each method call from a
director into PHP, and throw a C++ exception if an error occurred. This
exception can be caught in C++ to implement an error handler.
Currently no information about the PHP error is stored in the
Swig::DirectorMethodException object, but this will likely change in the
future.
</p>
<p>
It may be the case that a method call originates in PHP, travels up to
C++ through a proxy class, and then back into PHP via a director method.
If an exception occurs in PHP 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 PHP error state is still set when
Swig::DirectorMethodException is thrown, PHP will register the exception
as soon as the C wrapper function returns.
</p>
<H3><a name="Php_nn3_5"></a>29.3.5 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 PHP 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
PHP. Relative to the speed of PHP execution this is probably completely
negligible. For worst case routing, a method call that ultimately
resolves in C++ may take one extra detour through PHP in order to ensure
that the method does not have an extended PHP implementation. This could
result in a noticeable overhead in some cases.
</p>
<p>
Although directors make it natural to mix native C++ objects with PHP
objects (as director objects) via a common base class pointer, one
should be aware of the obvious fact that method calls to PHP 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 PHP.
</p>
<H3><a name="Php_nn3_6"></a>29.3.6 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>
<H3><a name="Php_nn3_7"></a>29.3.7 Miscellaneous</H3>
<p> Director typemaps for STL classes are mostly in place, and hence you
should be able to use std::string, etc., as you would any other type.
</p>
</body>
</html>