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git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk/SWIG@4141 626c5289-ae23-0410-ae9c-e8d60b6d4f22
218 lines
4.4 KiB
HTML
218 lines
4.4 KiB
HTML
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<title>SWIG:Examples:python:class</title>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/python/class/</tt>
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<hr>
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<H2>Wrapping a simple C++ class</H2>
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<tt>$Header$</tt><br>
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<p>
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This example illustrates the most primitive form of C++ class wrapping performed
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by SWIG. In this case, C++ classes are simply transformed into a collection of
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C-style functions that provide access to class members.
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<h2>The C++ Code</h2>
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Suppose you have some C++ classes described by the following (and admittedly lame)
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header file:
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<blockquote>
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<pre>
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/* File : example.h */
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class Shape {
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public:
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Shape() {
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nshapes++;
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}
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virtual ~Shape() {
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nshapes--;
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};
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double x, y;
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void move(double dx, double dy);
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virtual double area() = 0;
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virtual double perimeter() = 0;
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static int nshapes;
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};
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class Circle : public Shape {
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private:
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double radius;
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public:
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Circle(double r) : radius(r) { };
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virtual double area();
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virtual double perimeter();
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};
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class Square : public Shape {
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private:
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double width;
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public:
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Square(double w) : width(w) { };
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virtual double area();
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virtual double perimeter();
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};
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</pre>
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</blockquote>
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<h2>The SWIG interface</h2>
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A simple SWIG interface for this can be built by simply grabbing the header file
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like this:
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<blockquote>
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<pre>
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/* File : example.i */
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%module example
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%{
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#include "example.h"
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%}
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/* Let's just grab the original header file here */
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%include "example.h"
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</pre>
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</blockquote>
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Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this:
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<blockquote>
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<pre>
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% swig -c++ -python example.i
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</pre>
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</blockquote>
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<h2>A sample Python script</h2>
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Click <a href="example.py">here</a> to see a script that calls the C++ functions from Python.
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<h2>Key points</h2>
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<ul>
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<li>To create a new object, you call a constructor like this:
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<blockquote>
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<pre>
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c = example.new_Circle(10.0)
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</pre>
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</blockquote>
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<p>
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<li>To access member data, a pair of accessor functions are used.
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For example:
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<blockquote>
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<pre>
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example.Shape_x_set(c,15) # Set member data
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x = example.Shape_x_get(c) # Get member data
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</pre>
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</blockquote>
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Note: when accessing member data, the name of the class in which
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the member data was must be used. In this case, <tt>Shape_x_get()</tt>
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and <tt>Shape_x_set()</tt> are used since 'x' was defined in Shape.
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<p>
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<li>To invoke a member function, you simply do this
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<blockquote>
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<pre>
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print "The area is ", example.Shape_area(c)
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</pre>
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</blockquote>
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<p>
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<li>Type checking knows about the inheritance structure of C++. For example:
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<blockquote>
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<pre>
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example.Shape_area(c) # Works (c is a Shape)
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example.Circle_area(c) # Works (c is a Circle)
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example.Square_area(c) # Fails (c is definitely not a Square)
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</pre>
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</blockquote>
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<p>
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<li>To invoke a destructor, simply do this
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<blockquote>
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<pre>
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example.delete_Shape(c) # Deletes a shape
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</pre>
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</blockquote>
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(Note: destructors are currently not inherited. This might change later).
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<p>
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<li>Static member variables are wrapped as C global variables. For example:
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<blockquote>
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<pre>
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n = example.cvar.Shape_nshapes # Get a static data member
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example.cvar.Shapes_nshapes = 13 # Set a static data member
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</pre>
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</blockquote>
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</ul>
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<h2>General Comments</h2>
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<ul>
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<li>This low-level interface is not the only way to handle C++ code.
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Shadow classes provide a much higher-level interface.
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<p>
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<li>SWIG *does* know how to properly perform upcasting of objects in
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an inheritance hierarchy (including multiple inheritance). Therefore
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it is perfectly safe to pass an object of a derived class to any
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function involving a base class.
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<p>
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<li>A wide variety of C++ features are not currently supported by SWIG. Here is the
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short and incomplete list:
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<p>
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<ul>
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<li>Overloaded methods and functions. SWIG wrappers don't know how to resolve name
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conflicts so you must give an alternative name to any overloaded method name using the
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%name directive like this:
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<blockquote>
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<pre>
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void foo(int a);
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%name(foo2) void foo(double a, double b);
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</pre>
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</blockquote>
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<p>
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<li>Overloaded operators. Not supported at all. The only workaround for this is
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to write a helper function. For example:
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<blockquote>
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<pre>
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%inline %{
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Vector *vector_add(Vector *a, Vector *b) {
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... whatever ...
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}
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%}
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</pre>
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</blockquote>
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<p>
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<li>Namespaces. Not supported at all. Won't be supported until SWIG2.0 (if at all).
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<p>
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<li>Dave's snide remark: Like a large bottle of strong Tequilla, it's better to
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use C++ in moderation.
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</ul>
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<hr>
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</body>
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</html>
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