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  1. <html>
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  2. <head>
    3. 

  3. <title>SWIG:Examples:python:class</title>
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  4. </head>
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  5. 

  6. <body bgcolor="#ffffff">
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  7. 

  8. 

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

  10. <hr>
    11. 

  11. 

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

  13. 

  14. <tt>$Header$</tt><br>
    15. 

  15. 

  16. <p>
    17. 

  17. This example illustrates the most primitive form of C++ class wrapping performed
    18. 

  18. by SWIG.  In this case, C++ classes are simply transformed into a collection of
    19. 

  19. C-style functions that provide access to class members.
    20. 

  20. 

  21. <h2>The C++ Code</h2>
    22. 

  22. 

  23. Suppose you have some C++ classes described by the following (and admittedly lame) 
    24. 

  24. header file:
    25. 

  25. 

  26. <blockquote>
    27. 

  27. <pre>
    28. 

  28. /* File : example.h */
    29. 

  29. 

  30. class Shape {
    31. 

  31. public:
    32. 

  32.   Shape() {
    33. 

  33.     nshapes++;
    34. 

  34.   }
    35. 

  35.   virtual ~Shape() {
    36. 

  36.     nshapes--;
    37. 

  37.   };
    38. 

  38.   double  x, y;   
    39. 

  39.   void    move(double dx, double dy);
    40. 

  40.   virtual double area() = 0;
    41. 

  41.   virtual double perimeter() = 0;
    42. 

  42.   static  int nshapes;
    43. 

  43. };
    44. 

  44. 

  45. class Circle : public Shape {
    46. 

  46. private:
    47. 

  47.   double radius;
    48. 

  48. public:
    49. 

  49.   Circle(double r) : radius(r) { };
    50. 

  50.   virtual double area();
    51. 

  51.   virtual double perimeter();
    52. 

  52. };
    53. 

  53. 

  54. class Square : public Shape {
    55. 

  55. private:
    56. 

  56.   double width;
    57. 

  57. public:
    58. 

  58.   Square(double w) : width(w) { };
    59. 

  59.   virtual double area();
    60. 

  60.   virtual double perimeter();
    61. 

  61. };
    62. 

  62. </pre>
    63. 

  63. </blockquote>
    64. 

  64. 

  65. <h2>The SWIG interface</h2>
    66. 

  66. 

  67. A simple SWIG interface for this can be built by simply grabbing the header file
    68. 

  68. like this:
    69. 

  69. 

  70. <blockquote>
    71. 

  71. <pre>
    72. 

  72. /* File : example.i */
    73. 

  73. %module example
    74. 

  74. 

  75. %{
    76. 

  76. #include "example.h"
    77. 

  77. %}
    78. 

  78. 

  79. /* Let's just grab the original header file here */
    80. 

  80. %include "example.h"
    81. 

  81. </pre>
    82. 

  82. </blockquote>
    83. 

  83. 

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

  85. <blockquote>
    86. 

  86. <pre>
    87. 

  87. % swig -c++ -python example.i
    88. 

  88. </pre>
    89. 

  89. </blockquote>
    90. 

  90. 

  91. <h2>A sample Python script</h2>
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  92. 

  93. Click <a href="example.py">here</a> to see a script that calls the C++ functions from Python.
    94. 

  94. 

  95. <h2>Key points</h2>
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  96. 

  97. <ul>
    98. 

  98. <li>To create a new object, you call a constructor like this:
    99. 

  99. 

  100. <blockquote>
    101. 

  101. <pre>
    102. 

  102. c = example.new_Circle(10.0)
    103. 

  103. </pre>
    104. 

  104. </blockquote>
    105. 

  105. 

  106. <p>
    107. 

  107. <li>To access member data, a pair of accessor functions are used.
    108. 

  108. For example:
    109. 

  109. 

  110. <blockquote>
    111. 

  111. <pre>
    112. 

  112. example.Shape_x_set(c,15)    # Set member data
    113. 

  113. x = example.Shape_x_get(c)    # Get member data
    114. 

  114. </pre>
    115. 

  115. </blockquote>
    116. 

  116. 

  117. Note: when accessing member data, the name of the class in which
    118. 

  118. the member data was must be used.  In this case, <tt>Shape_x_get()</tt>
    119. 

  119. and <tt>Shape_x_set()</tt> are used since 'x' was defined in Shape.
    120. 

  120. 

  121. <p>
    122. 

  122. <li>To invoke a member function, you simply do this
    123. 

  123. 

  124. <blockquote>
    125. 

  125. <pre>
    126. 

  126. print "The area is ", example.Shape_area(c)
    127. 

  127. </pre>
    128. 

  128. </blockquote>
    129. 

  129. 

  130. <p>
    131. 

  131. <li>Type checking knows about the inheritance structure of C++. For example:
    132. 

  132. 

  133. <blockquote>
    134. 

  134. <pre>
    135. 

  135. example.Shape_area(c)       # Works (c is a Shape)
    136. 

  136. example.Circle_area(c)      # Works (c is a Circle)
    137. 

  137. example.Square_area(c)      # Fails (c is definitely not a Square)
    138. 

  138. </pre>
    139. 

  139. </blockquote>
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  140. 

  141. <p>
    142. 

  142. <li>To invoke a destructor, simply do this
    143. 

  143. 

  144. <blockquote>
    145. 

  145. <pre>
    146. 

  146. example.delete_Shape(c)     # Deletes a shape
    147. 

  147. </pre>
    148. 

  148. </blockquote>
    149. 

  149. 

  150. (Note: destructors are currently not inherited. This might change later).
    151. 

  151. 

  152. <p>
    153. 

  153. <li>Static member variables are wrapped as C global variables.  For example:
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  154. 

  155. <blockquote>
    156. 

  156. <pre>
    157. 

  157. n = example.cvar.Shape_nshapes     # Get a static data member
    158. 

  158. example.cvar.Shapes_nshapes = 13   # Set a static data member
    159. 

  159. </pre>
    160. 

  160. </blockquote>
    161. 

  161. 

  162. </ul>
    163. 

  163. 

  164. <h2>General Comments</h2>
    165. 

  165. 

  166. <ul>
    167. 

  167. <li>This low-level interface is not the only way to handle C++ code.
    168. 

  168. Shadow classes provide a much higher-level interface.
    169. 

  169. 

  170. <p>
    171. 

  171. <li>SWIG *does* know how to properly perform upcasting of objects in
    172. 

  172. an inheritance hierarchy (including multiple inheritance).  Therefore
    173. 

  173. it is perfectly safe to pass an object of a derived class to any
    174. 

  174. function involving a base class.
    175. 

  175. 

  176. <p>
    177. 

  177. <li>A wide variety of C++ features are not currently supported by SWIG.  Here is the
    178. 

  178. short and incomplete list:
    179. 

  179. 

  180. <p>
    181. 

  181. <ul>
    182. 

  182. <li>Overloaded methods and functions.  SWIG wrappers don't know how to resolve name
    183. 

  183. conflicts so you must give an alternative name to any overloaded method name using the
    184. 

  184. %name directive like this:
    185. 

  185. 

  186. <blockquote>
    187. 

  187. <pre>
    188. 

  188. void foo(int a);  
    189. 

  189. %name(foo2) void foo(double a, double b);
    190. 

  190. </pre>
    191. 

  191. </blockquote>
    192. 

  192. 

  193. <p>
    194. 

  194. <li>Overloaded operators.  Not supported at all. The only workaround for this is
    195. 

  195. to write a helper function. For example:
    196. 

  196. 

  197. <blockquote>
    198. 

  198. <pre>
    199. 

  199. %inline %{
    200. 

  200.     Vector *vector_add(Vector *a, Vector *b) {
    201. 

  201.           ... whatever ...
    202. 

  202.     }
    203. 

  203. %}
    204. 

  204. </pre>
    205. 

  205. </blockquote>
    206. 

  206. 

  207. <p>
    208. 

  208. <li>Namespaces.  Not supported at all. Won't be supported until SWIG2.0 (if at all).
    209. 

    210. 

  210. <p>
    211. 

  211. <li>Dave's snide remark: Like a large bottle of strong Tequilla, it's better to
    212. 

  212. use C++	in moderation.
    213. 

  213. 

  214. </ul>
    215. 

  215. 

  216. <hr>
    217. 

  217. </body>
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  218. </html>
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  219.