2.11.3 A Simple Example

The following example, provided as part of the GNAT examples, shows how to achieve procedural interfacing between Ada and C++ in both directions. The C++ class A has two methods. The first method is exported to Ada by the means of an extern C wrapper function. The second method calls an Ada subprogram. On the Ada side, The C++ calls are modelled by a limited record with a layout comparable to the C++ class. The Ada subprogram, in turn, calls the C++ method. So, starting from the C++ main program, the process passes back and forth between the two languages.

Here are the compilation commands:

     $ gnatmake -c simple_cpp_interface
     $ g++ -c cpp_main.C
     $ g++ -c ex7.C
     $ gnatbind -n simple_cpp_interface
     $ gnatlink simple_cpp_interface -o cpp_main --LINK=g++
           -lstdc++ ex7.o cpp_main.o

Here are the corresponding sources:

     
     //cpp_main.C
     
     #include "ex7.h"
     
     extern "C" {
       void adainit (void);
       void adafinal (void);
       void method1 (A *t);
     }
     
     void method1 (A *t)
     {
       t->method1 ();
     }
     
     int main ()
     {
       A obj;
       adainit ();
       obj.method2 (3030);
       adafinal ();
     }
     
     //ex7.h
     
     class Origin {
      public:
       int o_value;
     };
     class A : public Origin {
      public:
       void method1 (void);
       void method2 (int v);
       A();
       int   a_value;
     };
     
     //ex7.C
     
     #include "ex7.h"
     #include <stdio.h>
     
     extern "C" { void ada_method2 (A *t, int v);}
     
     void A::method1 (void)
     {
       a_value = 2020;
       printf ("in A::method1, a_value = %d \n",a_value);
     
     }
     
     void A::method2 (int v)
     {
        ada_method2 (this, v);
        printf ("in A::method2, a_value = %d \n",a_value);
     
     }
     
     A::A(void)
     {
        a_value = 1010;
       printf ("in A::A, a_value = %d \n",a_value);
     }

     -- Ada sources
     package body Simple_Cpp_Interface is
     
        procedure Ada_Method2 (This : in out A; V : Integer) is
        begin
           Method1 (This);
           This.A_Value := V;
        end Ada_Method2;
     
     end Simple_Cpp_Interface;
     
     with System;
     package Simple_Cpp_Interface is
        type A is limited
           record
              Vptr    : System.Address;
              O_Value : Integer;
              A_Value : Integer;
           end record;
        pragma Convention (C, A);
     
        procedure Method1 (This : in out A);
        pragma Import (C, Method1);
     
        procedure Ada_Method2 (This : in out A; V : Integer);
        pragma Export (C, Ada_Method2);
     
     end Simple_Cpp_Interface;