C function calling C++ member function - where the C code is compiled with a C compiler

Question

PLEASE before closing as dupe, read the question & see why it is different (hint: it's the C compiler)

I have Googled and found many, many, explanations of how a C function can call a C++ member function.

They all look similar to the accepted answer to this question, from a very high rep member.

It says

In a header file, put

extern "C" void* MyClass_create() {
   return new MyClass;
}
extern "C" void MyClass_release(void* myclass) {
   delete static_cast<MyClass*>(myclass);
}
extern "C" void MyClass_sendCommandToSerialDevice(void* myclass, int cmd, int params, int id) {
   static_cast<MyClass*>(myclass)->sendCommandToSerialDevice(cmd,params,id);
}

and, in the C code, put

void* myclass = MyClass_create();
MyClass_sendCommandToSerialDevice(myclass,1,2,3);
MyClass_release(myclass);

That seems straightforward, but what I don't understand is that the header file is going to have to reference MyClass (never mind that static_cast), but I want to compile my C code with a C compiler (gcc), not a C++ compiler (g++).

It won't work. How can I call a C++ member function from C code - which is compiled with a C compiler?

The other answer doesn't say "In a header file, put". It says "Then inside C++ perform the function call:". The C compiler can only process the declarations, not the function definitions. — Bo Persson
– Bo Persson, Commented Apr 21, 2017 at 8:29
Put only the declarations in the header file, the definitions of those functions need to be compiled by the C++ compiler. — Daniel Jour
– Daniel Jour, Commented Apr 21, 2017 at 8:30
The question you linked is marked as a duplicate of another question where there is a detailed example explaining what should be put into the header and what into the C++ definition — UnholySheep
– UnholySheep, Commented Apr 21, 2017 at 8:37
Ok, I didn't read all of your question, sorry. You can compile your C code with a C compiler, but will have to compile the C++ code with a C++ compiler. The C compiler can understand void* MyClass_create(); and call that function. If the C++ compiler sees the function as extern "C" void* MyClass_create(); it will make the call from C work. — Bo Persson
– Bo Persson, Commented Apr 21, 2017 at 8:37
The key point here is that you cannot call the member function with a C compiler. Maybe I should have been clearer, you implement a C++ function with a C interface that is callable from C, and that implementation does the C++ call. Note that it is inherently impossible to call a C++ member function from a C compiler, the compiler would have to parse the class and understand it, know how to call a member function... be a C++ compiler. — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Apr 21, 2017 at 10:31

yzt · Accepted Answer · 2017-04-21 08:43:08Z

5

You should do the following in C++:

In a C-compatible header file, e.g. interface.h, write:

#if defined(__cplusplus)
extern "C" {
#endif

void* MyClass_create();
void MyClass_release(void* myclass);
void MyClass_sendCommandToSerialDevice(void* myclass, int cmd, int params, int id);

#if defined(__cplusplus)
}
#endif

and in a source file, e.g. interface.cpp, put

/*extern "C"*/ void* MyClass_create() {
    return new MyClass;
}
/*extern "C"*/ void MyClass_release(void* myclass) {
    delete static_cast<MyClass*>(myclass);
}
/*extern "C"*/ void MyClass_sendCommandToSerialDevice(void* myclass, int cmd, int params, int id) {
    static_cast<MyClass*>(myclass)->sendCommandToSerialDevice(cmd,params,id);
}

Now, compile these either as part of the original C++ library, or a separate C++ library. You should be able to include the above .h file in your pure C programs and link them against the library.

answered Apr 21, 2017 at 8:43

yzt

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1 Comment

Mawg Over a year ago

A clear, excellent answer, which I have tested and am satisfied works (+1 and the answer).

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