TITLE: C Functions Calling C++

PROBLEM: What is the safest way for C functions to call C++ functions?

RESPONSE: Steve Clamage (steve@taumet.com), TauMetric Corp, 25 Jan 92


There is no guarantee that any C++ function uses the same calling
mechanism as any C function (or function in any other language).
A static function does not have a 'this' parameter, but any C++
function might have some sort of "hidden" parameter for any purpose.

Functions with no parameters and no return value eliminate several
kinds of complication with calling sequences, but there still remain
other problems.  For example, in typical implementations the calling
function and the called function have to agree on
  -  who removes the return address and frame pointer from the stack;
  -  which registers must be saved and restored;
  -  who does the register saving and restoring.

There is no rule that the C++ and C compilers have to do these things
the same way  (when you use an object-code C++ compiler).

The ONLY safe way to call a C++ function from C is to declare it 'extern
"C"'.  No member function may be so declared, so the only safe way to
call one is to write a wrapper function.

Example:

        class C { ... static void foo(); ... };
        extern "C" C_foo() { C::foo(); }

If the static member function exists so it can be called externally,
make it a friend function instead of a static member.  Then it may
be declared 'extern "C"' directly.