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  3. <title>SWIG:Examples:python:class</title>
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  7. 

  8. 

  9. <tt>SWIG/Examples/python/class/</tt>
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  10. <hr>
    11. 

  11. 

  12. <H2>Wrapping a simple C++ class</H2>
    13. 

  13. 

  14. <p>
    15. 

  15. This example illustrates the most primitive form of C++ class wrapping performed
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  16. by SWIG.  In this case, C++ classes are simply transformed into a collection of
    17. 

  17. C-style functions that provide access to class members.
    18. 

  18. 

  19. <h2>The C++ Code</h2>
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  20. 

  21. Suppose you have some C++ classes described by the following (and admittedly lame) 
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  22. header file:
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  23. 

  24. <blockquote>
    25. 

  25. <pre>
    26. 

  26. /* File : example.h */
    27. 

  27. 

  28. class Shape {
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  29. public:
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  30.   Shape() {
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  31.     nshapes++;
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  32.   }
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  33.   virtual ~Shape() {
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  34.     nshapes--;
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  35.   };
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  36.   double  x, y;   
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  37.   void    move(double dx, double dy);
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  38.   virtual double area() = 0;
    39. 

  39.   virtual double perimeter() = 0;
    40. 

  40.   static  int nshapes;
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  41. };
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  42. 

  43. class Circle : public Shape {
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  44. private:
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  45.   double radius;
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  46. public:
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  47.   Circle(double r) : radius(r) { };
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  48.   virtual double area();
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  49.   virtual double perimeter();
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  50. };
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  51. 

  52. class Square : public Shape {
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  53. private:
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  54.   double width;
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  55. public:
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  56.   Square(double w) : width(w) { };
    57. 

  57.   virtual double area();
    58. 

  58.   virtual double perimeter();
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  59. };
    60. 

  60. </pre>
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  61. </blockquote>
    62. 

  62. 

  63. <h2>The SWIG interface</h2>
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  64. 

  65. A simple SWIG interface for this can be built by simply grabbing the header file
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  66. like this:
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  67. 

  68. <blockquote>
    69. 

  69. <pre>
    70. 

  70. /* File : example.i */
    71. 

  71. %module example
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  72. 

  73. %{
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  74. #include "example.h"
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  75. %}
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  76. 

  77. /* Let's just grab the original header file here */
    78. 

  78. %include "example.h"
    79. 

  79. </pre>
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  80. </blockquote>
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  81. 

  82. Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this:
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  83. <blockquote>
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  84. <pre>
    85. 

  85. % swig -c++ -python example.i
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  86. </pre>
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  87. </blockquote>
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  88. 

  89. <h2>A sample Python script</h2>
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  90. 

  91. Click <a href="example.py">here</a> to see a script that calls the C++ functions from Python.
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  92. 

  93. <h2>Key points</h2>
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  94. 

  95. <ul>
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  96. <li>To create a new object, you call a constructor like this:
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  97. 

  98. <blockquote>
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  99. <pre>
    100. 

  100. c = example.new_Circle(10.0)
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  101. </pre>
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  102. </blockquote>
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  103. 

  104. <p>
    105. 

  105. <li>To access member data, a pair of accessor functions are used.
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  106. For example:
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  107. 

  108. <blockquote>
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  109. <pre>
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  110. example.Shape_x_set(c,15)    # Set member data
    111. 

  111. x = example.Shape_x_get(c)    # Get member data
    112. 

  112. </pre>
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  113. </blockquote>
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  114. 

  115. Note: when accessing member data, the name of the class in which
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  116. the member data was must be used.  In this case, <tt>Shape_x_get()</tt>
    117. 

  117. and <tt>Shape_x_set()</tt> are used since 'x' was defined in Shape.
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  118. 

  119. <p>
    120. 

  120. <li>To invoke a member function, you simply do this
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  121. 

  122. <blockquote>
    123. 

  123. <pre>
    124. 

  124. print "The area is ", example.Shape_area(c)
    125. 

  125. </pre>
    126. 

  126. </blockquote>
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  127. 

  128. <p>
    129. 

  129. <li>Type checking knows about the inheritance structure of C++. For example:
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  130. 

  131. <blockquote>
    132. 

  132. <pre>
    133. 

  133. example.Shape_area(c)       # Works (c is a Shape)
    134. 

  134. example.Circle_area(c)      # Works (c is a Circle)
    135. 

  135. example.Square_area(c)      # Fails (c is definitely not a Square)
    136. 

  136. </pre>
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  137. </blockquote>
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  138. 

  139. <p>
    140. 

  140. <li>To invoke a destructor, simply do this
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  141. 

  142. <blockquote>
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  143. <pre>
    144. 

  144. example.delete_Shape(c)     # Deletes a shape
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  145. </pre>
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  146. </blockquote>
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  147. 

  148. (Note: destructors are currently not inherited. This might change later).
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  149. 

  150. <p>
    151. 

  151. <li>Static member variables are wrapped as C global variables.  For example:
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  152. 

  153. <blockquote>
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  154. <pre>
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  155. n = example.cvar.Shape_nshapes     # Get a static data member
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  156. example.cvar.Shapes_nshapes = 13   # Set a static data member
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  157. </pre>
    158. 

  158. </blockquote>
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  159. 

  160. </ul>
    161. 

  161. 

  162. <h2>General Comments</h2>
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  163. 

  164. <ul>
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  165. <li>This low-level interface is not the only way to handle C++ code.
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  166. Proxy classes provide a much higher-level interface.
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  167. 

  168. <p>
    169. 

  169. <li>SWIG *does* know how to properly perform upcasting of objects in
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  170. an inheritance hierarchy (including multiple inheritance).  Therefore
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  171. it is perfectly safe to pass an object of a derived class to any
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  172. function involving a base class.
    173. 

  173. 

  174. <p>
    175. 

  175. <li>A wide variety of C++ features are not currently supported by SWIG.  Here is the
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  176. short and incomplete list:
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  177. 

  178. <p>
    179. 

  179. <ul>
    180. 

  180. <li>Overloaded methods and functions.  SWIG wrappers don't know how to resolve name
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  181. conflicts so you must give an alternative name to any overloaded method name using the
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  182. %name directive like this:
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  183. 

  184. <blockquote>
    185. 

  185. <pre>
    186. 

  186. void foo(int a);  
    187. 

  187. %name(foo2) void foo(double a, double b);
    188. 

  188. </pre>
    189. 

  189. </blockquote>
    190. 

  190. 

  191. <p>
    192. 

  192. <li>Overloaded operators.  Not supported at all. The only workaround for this is
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  193. to write a helper function. For example:
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  194. 

  195. <blockquote>
    196. 

  196. <pre>
    197. 

  197. %inline %{
    198. 

  198.     Vector *vector_add(Vector *a, Vector *b) {
    199. 

  199.           ... whatever ...
    200. 

  200.     }
    201. 

  201. %}
    202. 

  202. </pre>
    203. 

  203. </blockquote>
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  204. 

  205. <p>
    206. 

  206. <li>Namespaces.  Not supported at all. Won't be supported until SWIG2.0 (if at all).
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    208. 

  208. <p>
    209. 

  209. <li>Dave's snide remark: Like a large bottle of strong Tequilla, it's better to
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  210. use C++	in moderation.
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  211. 

  212. </ul>
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  213. 

  214. <hr>
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  215. </body>
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  216. </html>
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