Calling C++ class methods from C

[111106]

If you want to call class methods from C++ from within C, use a class wrapper.
One advantage of this method is that the C++ class remains unmodified, and can
even exist in a library.

First, let's define the C++ class, "Circle". For simplicity, we'll do everything
in a .h file, but it works just as well for classes declared in a .h and defined
in a .cpp file.

// Circle.h - a C++ class
 
#ifndef CIRCLE_H
#define CIRCLE_H
 
class Circle {
    public:
        Circle(float radius):_radius(radius) {}
        float getArea() { return 3.14159 * _radius * _radius; }
    private:
        float _radius;
};
 
#endif

Now let's declare a C++ wrapper for this class, which declares extern C methods,
which can be used from within C. This code must compile in both C++ and C files.
Use void * to point to class instances. Note the use of #ifdef __cplusplus.

/* Circle_C.h - must compile in both C and C++ */
 
#ifndef Circle_C_H
#define Circle_C_H
 
    #ifdef __cplusplus
    extern "C" {
    #endif
 
    extern void *Circle_C_new(float radius);
    extern void  Circle_C_delete(void *circle);
    extern float Circle_C_getArea(void *circle);
 
    #ifdef __cplusplus
    }
    #endif
 
#endif

Now define the extern functions. These will only be compiled as C++, so classes
can be referenced.

// Circle_C.cpp - extern C function definitions
 
#include "Circle_C.h"
#include "Circle.h"
 
extern void *Circle_C_new(float radius) {
    return new Circle(radius);
}
 
extern void Circle_C_delete(void *circle) {
    Circle *c = (Circle *)circle;
    delete c;
}
 
extern float Circle_C_getArea(void *circle) {
    Circle *c = (Circle *)circle;
    return c->getArea();
}

We can now use these extern C functions to access the C++ classes from C. Here
is a C main function example:

 /* mixed.c - a C file that accesses C++ methods */
 
#include <stdio.h>
#include "Circle_C.h"
 
void main() {
    float radius = 1.5;
 
    // Get a pointer to a Circle object
    void *circle = Circle_C_new(radius);
 
    // Pass the Circle object to the wrapper methods
    float area = Circle_C_getArea(circle);
 
    printf ("Circle of radius %f has area %f\n", radius, area);
 
    // Free the Circle object memory
    Circle_C_delete(circle);
}

Here is a sample make file which creates a "mixed" executable.

# Makefile - creates the executable "mixed".  Must link the stdc++ library
 
mixed: main.o Circle.o
    gcc -lstdc++ -o mixed main.o Circle_C.o
 
# Compile main as C
main.o: main.c
    gcc -c main.c -o main.o
 
# Compile Circle_C as C++
Circle_C.o: Circle_C.cpp
    g++ -c Circle_C.cpp -o Circle_C.o

Submitted by Jim Rogers

[prabu jayaraman]

Thanks a lot. It really helped for my work . YOU ROCKS....

[VFSAS]

GREAT! It really helped;