Question

In C or C++ what is the difference between function declaration and function signature?

I know something of function declaration but function signature is totally new to me. What is the point of having the concept of function signature? What are the two concepts used for actually?

Thanks!

Answer 1

A function declaration is the prototype for a function (or it can come from the function definition if no prototype has been seen by the compiler at that point) – it includes the return type, the name of the function and the types of the parameters (optionally in C).

A function signature is the parts of the function declaration that the compiler uses to perform overload resolution. Since multiple functions might have the same name (ie., they’re overloaded), the compiler needs a way to determine which of several possible functions with a particular name a function call should resolve to. The signature is what the compiler considers in that overload resolution. Specifically, the standard defines ‘signature’ as:

the information about a function that participates in overload resolution: the types of its parameters and, if the function is a class member, the cv-qualifiers (if any) on the function itself and the class in which the member function is declared.

Note that the return type is not part of the function signature. As the standard says in a footnote, “Function signatures do not include return type, because that does not participate in overload resolution”.