Question

Is the following code legal in C++?

void f(void* = 0)
{}

int main()
{
    f();
}

Which page of the C++ standard states that this usage is legal?

Answer 1

Yes, it’s legal.

There is no standard wording to allow this combination of features specifically; there simply isn’t any to disallow it, either.

Default argument syntax applies to function parameters in a parameter-declaration:

[C++11: 8.3.6/1]: If an initializer-clause is specified in a parameter-declaration this initializer-clause is used as a default argument. Default arguments will be used in calls where trailing arguments are missing.

…and function parameters in a parameter-declaration may be unnamed:

[C++11: 8.3.5/11]: [..] An identifier can optionally be provided as a parameter name. [..]

There is even an example of this usage under 8.3.6/4 (though examples are not normative text, so this cannot be used to prove anything concretely).