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  3. <title>SWIG:Examples:java:class</title>
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  8. 

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

  11. 

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

  14. <p>
    15. 

  15. This example illustrates the high level form of C++ class wrapping performed
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  16. by SWIG.  In this case, a C++ class has a proxy Java class, which 
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  17. provides access to C++ class members.
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  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>
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  25. <pre>
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  26. /* File : example.h */
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  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;
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  39.   virtual double perimeter() = 0;
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  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) { };
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  57.   virtual double area();
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  58.   virtual double perimeter();
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  59. };
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  60. </pre>
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  61. </blockquote>
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  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>
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  69. <pre>
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  70. /* File : example.i */
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  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 */
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  78. %include "example.h"
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  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>
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  85. % swig -c++ -java example.i
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  86. </pre>
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  87. </blockquote>
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  88. 

  89. <h2>A sample Java program</h2>
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  90. 

  91. Click <a href="runme.java">here</a> to see a Java program that calls the C++ functions from Java.
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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>
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  100. Circle c = new Circle(10);
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  101. </pre>
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  102. </blockquote>
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  103. 

  104. <p>
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  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. c.setX(15);        // Set member data
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  111. x = c.getX();      // Get member data
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  112. </pre>
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  113. </blockquote>
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  114. 

  115. <p>
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  116. <li>To invoke a member function, you simply do this
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  117. 

  118. <blockquote>
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  119. <pre>
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  120. System.out.println( "The area is " + c.area() );
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  121. </pre>
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  122. </blockquote>
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  123. 

  124. <p>
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  125. <li>To invoke a destructor, simply do this
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  126. 

  127. <blockquote>
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  128. <pre>
    129. 

  129. c.delete();     // Deletes a shape
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  130. </pre>
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  131. </blockquote>
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  132. 

  133. <p>
    134. 

  134. <li>Static member variables are wrapped with java static get and set access functions.  For example:
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  135. 

  136. <blockquote>
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  137. <pre>
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  138. n = Shape.getNshapes();     // Get a static data member
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  139. Shape.setNshapes(13);       // Set a static data member
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  140. </pre>
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  141. </blockquote>
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  142. 

  143. </ul>
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  144. 

  145. <h2>General Comments</h2>
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  146. 

  147. <ul>
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  148. <li>This high-level interface using proxy classes is not the only way to handle C++ code. 
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  149. A low level interface using c functions to access member variables and member functions is the alternative SWIG
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  150. approach. This entails passing around the c pointer or c++ 'this' pointer and as such it is not difficult to crash the JVM. 
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  151. The abstraction of the underlying pointer by the java proxy classes far better fits the java programming paradigm.
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  152. 

  153. <p>
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  154. <li>SWIG *does* know how to properly perform upcasting of objects in an inheritance
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  155. hierarchy (including multiple inheritance).  However Java classes can only derive from one base class so multiple inheritance
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  156. is not implemented. Java classes can implement more than one interface so there is scope for improvement in the future. 
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  157. 

  158. <p>
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  159. <li>A wide variety of C++ features are not currently supported by SWIG.  Here is the
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  160. short and incomplete list:
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  161. 

  162. <p>
    163. 

  163. <ul>
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  164. <li>Overloaded methods and functions.  SWIG wrappers don't know how to resolve name
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  165. conflicts so you must give an alternative name to any overloaded method name using the
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  166. %name directive like this:
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  167. 

  168. <blockquote>
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  169. <pre>
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  170. void foo(int a);  
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  171. %name(foo2) void foo(double a, double b);
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  172. </pre>
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  173. </blockquote>
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  174. 

  175. <p>
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  176. <li>Overloaded operators.  Not supported at all. The only workaround for this is
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  177. to write a helper function. For example:
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  178. 

  179. <blockquote>
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  180. <pre>
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  181. %inline %{
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  182.     Vector *vector_add(Vector *a, Vector *b) {
    183. 

  183.           ... whatever ...
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  184.     }
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  185. %}
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  186. </pre>
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  187. </blockquote>
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  188. 

  189. <p>
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  190. <li>Namespaces.  Not supported at all. Won't be supported until SWIG2.0 (if at all).
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    192. 

  192. </ul>
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  193. </ul>
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  194. 

  195. <hr>
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  196. </body>
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