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<html>
<head>
<title>SWIG:Examples:python:class</title>
</head>
<body bgcolor="#ffffff">
<tt>SWIG/Examples/python/class/</tt>
<hr>
<H2>Wrapping a simple C++ class</H2>
<tt>$Header$</tt><br>
<p>
This example illustrates the most primitive form of C++ class wrapping performed
by SWIG. In this case, C++ classes are simply transformed into a collection of
C-style functions that provide access to class members.
<h2>The C++ Code</h2>
Suppose you have some C++ classes described by the following (and admittedly lame)
header file:
<blockquote>
<pre>
/* File : example.h */
class Shape {
public:
Shape() {
nshapes++;
}
virtual ~Shape() {
nshapes--;
};
double x, y;
void move(double dx, double dy);
virtual double area() = 0;
virtual double perimeter() = 0;
static int nshapes;
};
class Circle : public Shape {
private:
double radius;
public:
Circle(double r) : radius(r) { };
virtual double area();
virtual double perimeter();
};
class Square : public Shape {
private:
double width;
public:
Square(double w) : width(w) { };
virtual double area();
virtual double perimeter();
};
</pre>
</blockquote>
<h2>The SWIG interface</h2>
A simple SWIG interface for this can be built by simply grabbing the header file
like this:
<blockquote>
<pre>
/* File : example.i */
%module example
%{
#include "example.h"
%}
/* Let's just grab the original header file here */
%include "example.h"
</pre>
</blockquote>
Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this:
<blockquote>
<pre>
% swig -c++ -python example.i
</pre>
</blockquote>
<h2>A sample Python script</h2>
Click <a href="example.py">here</a> to see a script that calls the C++ functions from Python.
<h2>Key points</h2>
<ul>
<li>To create a new object, you call a constructor like this:
<blockquote>
<pre>
c = example.new_Circle(10.0)
</pre>
</blockquote>
<p>
<li>To access member data, a pair of accessor functions are used.
For example:
<blockquote>
<pre>
example.Circle_x_set(c,15) # Set member data
x = example.Shape_x_get(c) # Get member data
</pre>
</blockquote>
Note: when accessing member data, the name of the base class or the derived class can be
used in the function name as shown above. Of course, it would probably be more
proper to just use the base class version such as <tt>Shape_x_get()</tt>
<p>
<li>To invoke a member function, you simply do this
<blockquote>
<pre>
print "The area is ", example.Shape_area(c)
</pre>
</blockquote>
<p>
<li>Type checking knows about the inheritance structure of C++. For example:
<blockquote>
<pre>
example.Shape_area(c) # Works (c is a Shape)
example.Circle_area(c) # Works (c is a Circle)
example.Square_area(c) # Fails (c is definitely not a Square)
</pre>
</blockquote>
<p>
<li>To invoke a destructor, simply do this
<blockquote>
<pre>
example.delete_Shape(c) # Deletes a shape
</pre>
</blockquote>
(Note: destructors are currently not inherited. This might change later).
<p>
<li>Static member variables are wrapped as C global variables. For example:
<blockquote>
<pre>
n = example.cvar.Shape_nshapes # Get a static data member
example.cvar.Shapes_nshapes = 13 # Set a static data member
</pre>
</blockquote>
</ul>
<h2>General Comments</h2>
<ul>
<li>This low-level interface is not the only way to handle C++ code. Shadow classes
provide a much higher-level interface.
<p>
<li>SWIG *does* know how to properly perform upcasting of objects in an inheritance
hierarchy (including multiple inheritance). Therefore it is perfectly safe to pass
an object of a derived class to any function involving a base class.
<p>
<li>A wide variety of C++ features are not currently supported by SWIG. Here is the
short and incomplete list:
<p>
<ul>
<li>Overloaded methods and functions. SWIG wrappers don't know how to resolve name
conflicts so you must give an alternative name to any overloaded method name using the
%name directive like this:
<blockquote>
<pre>
void foo(int a);
%name(foo2) void foo(double a, double b);
</pre>
</blockquote>
<p>
<li>Overloaded operators. Not supported at all. The only workaround for this is
to write a helper function. For example:
<blockquote>
<pre>
%inline %{
Vector *vector_add(Vector *a, Vector *b) {
... whatever ...
}
%}
</pre>
</blockquote>
<p>
<li>Namespaces. Not supported at all. Won't be supported until SWIG2.0 (if at all).
<p>
<li>Templates. Not supported at all. SWIG throws out anything that looks like a template.
You can work around the problem by aliasing a template class behind a typedef however.
For example:
<blockquote>
<pre>
%{
typedef vector<int> IntVector;
%}
class IntVector {
public:
... methods ...
};
</pre>
</blockquote>
</ul>
<p>
<li>There is no guarantee that an extremely complex C++ application will be able to compile
as a Python extension. Sorry.
<p>
<li>Dave's snide remark: Like a large bottle of strong Tequilla, it's better to
use C++ in moderation.
</ul>
<hr>
</body>
</html>