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// ---- .h  -------
// when this header is included from a .cpp file, you have both the C++ and C API available
// including from a .c file gives you the C API only
// C++ API declarations
#ifdef __cplusplus   // compiled as C++ ==>
class CFoo 
{
  public:
    int Foo() { return ... };
    static CFoo * CreateInstance() { return new CFoo; }
};

#endif               // <== compiled as C++

// C API declarations
#ifdef __cplusplus   
extern "C" { // when compiling with C++, we explicitely say this has "C" linkage ==> 
#endif
void * FooCreate();
int FoFoo(void *);
#ifdef __cplusplus
} // extern "C"  // <== C linkage ends here 

#endif

// ---- .cpp ----------
// this file implements both the C++ and C API
// normal C++ implementation - in this example nothing, cause it's all in the header

// Now, implementing the functions with C linkage
// note that, though the functions are declared "extern "C", you can call C++ stuff
extern "C" void * FooCreate() { return (void *) CFoo::CreateInstance(); }
extern "C" int FooFoo(void * f) { return ((CFoo *)f)->Foo(); }