Can an unnamed parameter of function have a default value?

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).

Answer 2

Yes, it's perfectly legal. An obvious example is found in N3485 8.3.6 Default Arguments/4:

[Example: the declaration

void point(int = 3, int = 4);  

declares a function that can be called with zero, one, or two arguments of type int.

Answer 3

Yes, it is legal.
The syntax productions given for function parameters in clause 8.3.5/1 allow a parameter declaration without an identifier, but with an assignment expression (as initialiser).

Answer 4

Not only is it legal, it could actually be quite useful depending on your coding style.

Default parameters are only meaningful in a function declaration.

Named parameters are only meaningful in a function definition.

f.h:

void f(void*=nullptr);

f.cc

void f(void* x)
{
...
}