C++ function types

Here’s the relevant paragraph from the Standard. It pretty much speaks for itself.

8.3.5/10

A typedef of function type may be used to declare a function but shall not be used to define a function (8.4).

Example:

typedef void F();
F  fv;         // OK: equivalent to void fv();
F  fv { }      // ill-formed
void fv() { }  // OK: definition of fv

A typedef of a function type whose declarator includes a cv-qualifier-seq shall be used only to declare the function type for a non-static member function, to declare the function type to which a pointer to member refers, or to declare the top-level function type of another function typedef declaration.

Example:

typedef int FIC(int) const;
FIC f;               // ill-formed: does not declare a member function
struct S {
  FIC f;             // OK
};
FIC S::*pm = &S::f;  // OK