FAQ 36.05 How can an object of a C class be passed to or from a C function?

FAQ 36.05 How can an object of a C++ class be passed to or from a C function?

With a layer of glue code.

The example that follows is a bilingual header file, readable by both a straight C compiler and a C++ compiler. Bilingual header files often use the preprocessor symbol __cplusplus, which is automatically #defined by C++ compilers but left undefined by C compilers.

 /****** Bilingual C/C++ header file: Fred.h ******/ #ifdef __cplusplus   class Fred {   public:     Fred() throw();     void wilma(int i) throw();   protected:     int i_;   };   inline Fred::Fred() throw() : i_(0) { }   inline void Fred::wilma(int i) throw() { i_ = i; } #else   struct Fred {     int i_;   };   typedef  struct Fred  Fred; #endif #ifdef __cplusplus   extern "C" { #endif extern void cppCallingC(Fred* p); extern void cCallingCpp(Fred* p, int param); #ifdef __cplusplus   } #endif 

The function cCallingCpp() might be defined in a C++ file, such as c++-code.cpp.

 // This is C++ code #include "Fred.h" void cCallingCpp(Fred* p, int param) throw() { p->wilma(param); } 

The function cppCallingC() might be defined in a C file, such as c-code.c.

 /* This is C code */ #include "Fred.h" void cppCallingC(Fred* p) { cCallingCpp(p, 3); } 

A Fred might be passed to the C code by main() (recall that main() must be compiled by the C++ compiler).

 // This is C++ code #include "Fred.h" int main() {   Fred x;   cppCallingC(&x); } 

Note that C code should not cast pointers that refer to C++ objects because doing so can introduce errors, especially in cases where the pointer is returned to C++. For example, most compilers adjust the pointer during certain pointer casts involving multiple and/or virtual inheritance; the C compiler doesn't know how to do these adjustments.

The example assumes that the C compiler supports ANSI-C function prototypes. Use #ifdef __STDC__ for those rare legacy situations that require selecting code that supports only the outdated K&R C style.