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  3<title>SWIG:Examples:python:class</title>
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  9<tt>SWIG/Examples/python/class/</tt>
 10<hr>
 11
 12<H2>Wrapping a simple C++ class</H2>
 13
 14<p>
 15This example illustrates the most primitive form of C++ class wrapping performed
 16by SWIG.  In this case, C++ classes are simply transformed into a collection of
 17C-style functions that provide access to class members.
 18
 19<h2>The C++ Code</h2>
 20
 21Suppose you have some C++ classes described by the following (and admittedly lame) 
 22header file:
 23
 24<blockquote>
 25<pre>
 26/* File : example.h */
 27
 28class Shape {
 29public:
 30  Shape() {
 31    nshapes++;
 32  }
 33  virtual ~Shape() {
 34    nshapes--;
 35  };
 36  double  x, y;   
 37  void    move(double dx, double dy);
 38  virtual double area() = 0;
 39  virtual double perimeter() = 0;
 40  static  int nshapes;
 41};
 42
 43class Circle : public Shape {
 44private:
 45  double radius;
 46public:
 47  Circle(double r) : radius(r) { };
 48  virtual double area();
 49  virtual double perimeter();
 50};
 51
 52class Square : public Shape {
 53private:
 54  double width;
 55public:
 56  Square(double w) : width(w) { };
 57  virtual double area();
 58  virtual double perimeter();
 59};
 60</pre>
 61</blockquote>
 62
 63<h2>The SWIG interface</h2>
 64
 65A simple SWIG interface for this can be built by simply grabbing the header file
 66like this:
 67
 68<blockquote>
 69<pre>
 70/* File : example.i */
 71%module example
 72
 73%{
 74#include "example.h"
 75%}
 76
 77/* Let's just grab the original header file here */
 78%include "example.h"
 79</pre>
 80</blockquote>
 81
 82Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this:
 83<blockquote>
 84<pre>
 85% swig -c++ -python example.i
 86</pre>
 87</blockquote>
 88
 89<h2>A sample Python script</h2>
 90
 91Click <a href="example.py">here</a> to see a script that calls the C++ functions from Python.
 92
 93<h2>Key points</h2>
 94
 95<ul>
 96<li>To create a new object, you call a constructor like this:
 97
 98<blockquote>
 99<pre>
100c = example.new_Circle(10.0)
101</pre>
102</blockquote>
103
104<p>
105<li>To access member data, a pair of accessor functions are used.
106For example:
107
108<blockquote>
109<pre>
110example.Shape_x_set(c,15)    # Set member data
111x = example.Shape_x_get(c)    # Get member data
112</pre>
113</blockquote>
114
115Note: when accessing member data, the name of the class in which
116the member data was must be used.  In this case, <tt>Shape_x_get()</tt>
117and <tt>Shape_x_set()</tt> are used since 'x' was defined in Shape.
118
119<p>
120<li>To invoke a member function, you simply do this
121
122<blockquote>
123<pre>
124print "The area is ", example.Shape_area(c)
125</pre>
126</blockquote>
127
128<p>
129<li>Type checking knows about the inheritance structure of C++. For example:
130
131<blockquote>
132<pre>
133example.Shape_area(c)       # Works (c is a Shape)
134example.Circle_area(c)      # Works (c is a Circle)
135example.Square_area(c)      # Fails (c is definitely not a Square)
136</pre>
137</blockquote>
138
139<p>
140<li>To invoke a destructor, simply do this
141
142<blockquote>
143<pre>
144example.delete_Shape(c)     # Deletes a shape
145</pre>
146</blockquote>
147
148(Note: destructors are currently not inherited. This might change later).
149
150<p>
151<li>Static member variables are wrapped as C global variables.  For example:
152
153<blockquote>
154<pre>
155n = example.cvar.Shape_nshapes     # Get a static data member
156example.cvar.Shapes_nshapes = 13   # Set a static data member
157</pre>
158</blockquote>
159
160</ul>
161
162<h2>General Comments</h2>
163
164<ul>
165<li>This low-level interface is not the only way to handle C++ code.
166Proxy classes provide a much higher-level interface.
167
168<p>
169<li>SWIG *does* know how to properly perform upcasting of objects in
170an inheritance hierarchy (including multiple inheritance).  Therefore
171it is perfectly safe to pass an object of a derived class to any
172function involving a base class.
173
174<p>
175<li>A wide variety of C++ features are not currently supported by SWIG.  Here is the
176short and incomplete list:
177
178<p>
179<ul>
180<li>Overloaded methods and functions.  SWIG wrappers don't know how to resolve name
181conflicts so you must give an alternative name to any overloaded method name using the
182%name directive like this:
183
184<blockquote>
185<pre>
186void foo(int a);  
187%name(foo2) void foo(double a, double b);
188</pre>
189</blockquote>
190
191<p>
192<li>Overloaded operators.  Not supported at all. The only workaround for this is
193to write a helper function. For example:
194
195<blockquote>
196<pre>
197%inline %{
198    Vector *vector_add(Vector *a, Vector *b) {
199          ... whatever ...
200    }
201%}
202</pre>
203</blockquote>
204
205<p>
206<li>Namespaces.  Not supported at all. Won't be supported until SWIG2.0 (if at all).
207
208<p>
209<li>Dave's snide remark: Like a large bottle of strong Tequilla, it's better to
210use C++	in moderation.
211
212</ul>
213
214<hr>
215</body>
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