How to call C++ function from C?

Question

I know this.

Calling C function from C++:

If my application was in C++ and I had to call functions from a library written in C. Then I would have used

//main.cpp

extern "C" void C_library_function(int x, int y);//prototype
C_library_function(2,4);// directly using it.

This wouldn't mangle the name C_library_function and linker would find the same name in its input *.lib files and problem is solved.

Calling C++ function from C???

But here I'm extending a large application which is written in C and I need to use a library which is written in C++. Name mangling of C++ is causing trouble here. Linker is complaining about the unresolved symbols. Well I cannot use C++ compiler over my C project because thats breaking lot of other stuff. What is the way out?

By the way I'm using MSVC

Answer 1

You need to create a C API for exposing the functionality of your C++ code. Basically, you will need to write C++ code that is declared extern "C" and that has a pure C API (not using classes, for example) that wraps the C++ library. Then you use the pure C wrapper library that you've created.

Your C API can optionally follow an object-oriented style, even though C is not object-oriented. Ex:

 // *.h file
 // ...
 #ifdef __cplusplus
 #define EXTERNC extern "C"
 #else
 #define EXTERNC
 #endif

 typedef void* mylibrary_mytype_t;

 EXTERNC mylibrary_mytype_t mylibrary_mytype_init();
 EXTERNC void mylibrary_mytype_destroy(mylibrary_mytype_t mytype);
 EXTERNC void mylibrary_mytype_doit(mylibrary_mytype_t self, int param);

 #undef EXTERNC
 // ...


 // *.cpp file
 mylibrary_mytype_t mylibrary_mytype_init() {
   return new MyType;
 }

 void mylibrary_mytype_destroy(mylibrary_mytype_t untyped_ptr) {
    MyType* typed_ptr = static_cast<MyType*>(untyped_ptr);
    delete typed_ptr;
 }

 void mylibrary_mytype_doit(mylibrary_mytype_t untyped_self, int param) {
    MyType* typed_self = static_cast<MyType*>(untyped_self);
    typed_self->doIt(param);
 }