parameter name omitted, C++ vs C

Question

In C++, I tend to omit the parameter's name under some circumstances. But in C, I got an error when I omitted the parameter's name.

Here is the code:

void foo(int);  //forward-decl, it's OK to omit the parameter's name, in both C++ and C

int main()
{
    foo(0);
    return 0;
}

void foo(int)  //definition in C, it cannot compile with gcc
{
    printf("in foo
");
}

void foo(int)  //definition in C++, it can compile with g++
{
    cout << "in foo" << endl;
}

Why is that? Can't I omit the parameter's name in C function definition?

Answer 1

No, in C you cannot omit identifiers for parameters in function definitions.

The C99 standard says:

[6.9.1.5] If the declarator includes a parameter type list, the declaration of each parameter shall include an identifier, except for the special case of a parameter list consisting of a single parameter of type void, in which case there shall not be an identifier. No declaration list shall follow.

The C++14 standard says:

[8.3.5.11] An identifier can optionally be provided as a parameter name; if present in a function definition , it names a parameter (sometimes called “formal argument”). [Note: In particular, parameter names are also optional in function definitions and names used for a parameter in different declarations and the definition of a function need not be the same.]