C++ Function declare in function

Question

Why does this compile?

int main() {
    void f();
}

What is f? Is it a function or a variable and how can I use it?

Answer 1

You have a function declaration. It's OK to declare functions inside a block scope. It's just not possible to define a function in a block. Declaring a function only locally is perfectly fine, e.g. when you want to refer to some other, existing function:

int main()
{
    int foo();
    return foo();
}

You just have to link this with some translation unit that defines int foo();.

(It's probably just not very good practice to do this, since it hides the dependency of your code on the other function in a way that's almost impossible to see without careful reading of the code.)

Note that the local declaration hides other overloads of the function, so argument lookup may be dif­fer­ent from if you had declared the function at the global scope. See @David Rodríguez's vastly more de­tailed answer.