Possible ambiguity with extern “C”, overloading, and function pointers

Question

With normal functions, one can write

extern \"C\" int Frotz(int);  // in a header

int Frotz(int x) { return x; }

With function pointers, h

Answer 1

The C ABI and the C++ ABI are not guaranteed to be the same. So, an extern "C" function pointer is a different type from a C++ function pointer. You need something like this:

extern "C" {
    typedef int (*KlutzFuncType)(int);
    int Klutz (KlutzFuncType, int);
}

int Klutz (KlutzFuncType fptr, int x) { return (*fptr)(x); }

There is some discussion of this issue here.

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I only have a copy of the draft. From 7.5p1:

Two function types with different language linkages are distinct types even if they are otherwise identical.

My reading of this is that the first parameter of your first Klutz has a different type than the first parameter of your second Klutz, and so your second Klutz should have C++ linkage.

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There are C++ implementations that do not take language linkage into account for function types, despite what the standard says. In the following code snippet, KlutzCxxFuncType refers to a function with C++ linkage, while KlutzCFuncType refers to a function with C linkage.

typedef int (*KlutzCxxFuncType)(int);

extern "C" {
    typedef int (*KlutzCFuncType)(int);
    int Klutz (KlutzCFuncType, int);
}

int Klutz (KlutzCxxFuncType fptr, int x) { return (*fptr)(x); }
int Klutz (KlutzCFuncType fptr, int x) { return (*fptr)(x); }

A compiler that does not distinguish function types based on language linkage will generate a redefinition error on this code. For example, g++ 4.7.2 will emit:

prog.cpp: In function ‘int Klutz(KlutzCFuncType, int)’:
prog.cpp:9:5: error: redefinition of ‘int Klutz(KlutzCFuncType, int)’
prog.cpp:8:5: error: ‘int Klutz(KlutzCxxFuncType, int)’ previously defined here