what is the difference between function declaration and signature?

c++ c function declaration signature
Tim picture Tim · Feb 24, 2010 · Viewed 44.9k times · Source

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

Michael Burr picture Michael Burr · Feb 24, 2010

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".

Related questions

Is it safe to return a struct in C or C++?

What I understand is that this shouldn't be done, but I believe I've seen examples that do something like this (note code is not necessarily syntactically correct but the idea is there) typedef struct{ int a,b; }mystruct; And then …

Read More
Declare variables at top of function or in separate scopes?

Which is preferred, method 1 or method 2? Method 1: LRESULT CALLBACK wpMainWindow(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam) { switch (msg) { case WM_PAINT: { HDC hdc; PAINTSTRUCT ps; RECT rc; GetClientRect(hwnd, &rc); hdc = BeginPaint(hwnd, &ps); // drawing here …

Read More
Understanding the difference between f() and f(void) in C and C++ once and for all

Ok, so I have heard different opinions on this subject and just want to make sure I understand it correctly. For C++ Declarations void f(); and void f(void); mean precisely the same thing, the function f does not take …

Read More