Using .c and .cpp files in Visual Studio at the same time

Question

Trying to figure out how to get an application to compile that uses both C and C++ files. Not the entire code, but enough to get the idea:

main.cpp:

#include <windows.h>
#include <stdio.h>
#include <string.h>
#include "one.h"
#include "two.h"

int __stdcall WinMain(HINSTANCE hInst, HINSTANCE hInst2, LPSTR lpCmdLine, int nShowCmd) {
    FunctionOne();
    FunctionTwo();
}

one.cpp:

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <gdiplus.h>
#include <gdiplusflat.h>
using namespace Gdiplus;
using namespace Gdiplus::DllExports;

int FunctionOne() {
}

two.c

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>

int FunctionTwo() {
}

The header files contain only definitions for those functions.

Now, if I compile this with a main.cpp, I get an "unresolved external symbol" for FunctionTwo. If I compile this with a main.c, I get the same thing for FunctionOne. Is this even possible, and if so, how would I set up the project to compile properly (Visual Studio 2010)?

It compiles fine if I comment out the alternate function depending on the extension for main.

Thanks!

See this -> stackoverflow.com/questions/12573816/… - MSVS compiles .c files as C and .cpp files as C++. — Luchian Grigore
– Luchian Grigore, Commented Sep 27, 2012 at 21:25

Hans Passant · Accepted Answer · 2012-09-27 21:37:32Z

12

The problem is two.h, it almost certainly wasn't written to allow a C++ compiler to properly compile the C function prototype. You'll want to take advantage of the predefined __cplusplus macro, like this:

two.h:

#ifdef __cplusplus
extern "C" {
#endif

int FunctionTwo();
// etc...

#ifdef __cplusplus
}
#endif

Lovely macro soup ;) If the header is pre-baked and never saw a C++ compiler before then do this in your .cpp source code file:

#include <windows.h>
#include <stdio.h>
#include <string.h>
#include "one.h"
extern "C" {
#include "two.h"
}

Some programmers name their header files .hpp if they contain C++ declarations and .h if they contain C declarations. That's a pretty good practice I personally favor. So does the Boost team. It didn't otherwise set the world on fire.

edited Sep 27, 2012 at 21:37

answered Sep 27, 2012 at 21:30

Hans Passant

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1 Comment

Fmstrat Over a year ago

Thank you for the response. This was marked as a duplicate, but I was unable to locate the other answer as it didn't focus on the wording of the question in my mind (C vs. C++ instead of the message itself). Your solution worked for me.

Seth Carnegie · Accepted Answer · 2012-09-27 21:25:44Z

0

C++ does name-mangling to support function overloading while C does not. You will have to mark your function as extern "C" to prevent name mangling.

// main.cpp

extern "C" int FunctionTwo();

.. the rest ..

// two.c

extern "C" int FunctionTwo() {
    // stuff
}

See http://www.parashift.com/c++-faq/mixing-c-and-cpp.html for more information on mixing C and C++.

answered Sep 27, 2012 at 21:25

Seth Carnegie

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1 Comment

Luchian Grigore Over a year ago

Why not just FunctionTwo? FunctionOne is compiled as C++, no?

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