Why is this variadic function ambiguous?

Question

This is related to my earlier post. I\'d like to know why one attempted solution didn\'t work.

template              /* A */
size_t num_         


        

Answer 1

I don't understand how this is ambiguous -- A is a declaration and B is a definition of the function declared by A. Right?

No. A is a declaration of a function template, and B is a declaration (and definition) of another function template.

The compiler has no way to decide between the two: they both have no arguments, and the template arguments are a match for both.

The one in the middle is an explicit total specialization of the function template declared in A.

If you tried to make B another specialization of A:

template  /* B */
size_t num_args()
{
    return 1 + num_args  ();
}

... you'd end up with a partial specialization of a function template, which is not allowed.

You can do this with the usual trick of using a class template with partial specializations and a function template that calls into the class template:

template 
class Num_Args;

template <>
struct Num_Args <>
{
    static constexpr size_t calculate() {
        return 0;
    }
};

template 
struct Num_Args 
{
    static constexpr size_t calculate() {
        return 1 + Num_Args::calculate();
    }
};

template  /* B */
constexpr size_t num_args ()
{
    return Num_Args::calculate();
}