Question

I have encountered a working (with XLC8 and MSFT9 compilers) piece of code, containing a C++ file with a function defined with C linkage and a reference argument. This bugs me, as references are C++ only. The function in question is called from C code, where it is declared as taking a pointer argument to the same type in place of the reference argument.

Simplified example:

C++ file:

extern "C" void f(int &i)
{
    i++;
}

C file:

void f(int *);

int main()
{
    int a = 2;
    f(&a);
    printf("%d\n", a);  /* Prints 3 */
}

Now, the word on the street is that most C++ compilers, under the hood, implement references just like a pointer. Is it like that and just pure luck the reason this code works or does it say somewhere in the C++ specification what the result is when you define a function with a reference argument and C linkage? I haven’t been able to find any information on this.

Answer 1

My copy of n3000.pdf (from here), has this to say in section 7.5—Linkage specifications:

9. Linkage from C++ to objects defined in
other languages and to objects defined
in C++ from other languages is
implementation-defined and
language-dependent. Only where the
object layout strategies of two
language implementations are similar
enough can such linkage be achieved.

Since C and C++ are different languages, this means that you can’t rely on this "feature" of common compilers.

Stronger is note 5 in the same section (emphasis mine):

If two declarations declare functions
with the same name and
parameter-type-list (8.3.5) to be
members of the same namespace or
declare objects with the same name to
be members of the same namespace and
the declarations give the names
different language linkages, the
program is ill-formed; no diagnostic
is required if the declarations appear
in different translation units.

So, I would say that what you did is not guaranteed to work according to the standard, and the compiler is not required to print a diagnostic for the example you have given because the declarations are in different translation units.

FYI, it "works for me" with gcc and g++ version 4.2.1 on Snow Leopard.