definition of static class function outside of class definition

Question

Consider the followin simple class definition -

    // file A.h
#include <iostream>
class A {
public:
  static int f();
  static const int aa;
};


    // file A.cpp
#include "a.h"
using namespace std;
const   int A::aa = 10;
int A::f() {
    return A::aa;
}

And this is my main file -

    // main.cpp file
#include "a.h"
#include "b.h"
using namespace std;
const int A::aa = 100;
int A::f();
int main() {
    cout << A::aa << "\n";
    cout << A::f() << "\n";
}

When I try to compile main.cpp, the compiler complains that the declaration of A::f() in main.cpp outside the class is a declaration, not a definition. Why is this? I do not intend to define A::f() in main.cpp. It is defined in A.cpp and the linker should link the declaration of A::f() in main.cpp with its definition in A.cpp. So I do not understand why am I getting this error. Note this is a compilation error.

Answer 1

C++11 standard §9.3 [class.mftc] p3:

[...] Except for member function definitions that appear outside of a class definition, and except for explicit specializations of member functions of class templates and member function templates (14.7) appearing outside of the class definition, a member function shall not be redeclared.

Aside from that, you'll get a linker error due to multiple definitions of A::aa, but it seems that you expected that, judging from your last sentence.

Answer 2

The class and it's members are already defined, you just have to include the file into your main (which you've done). You do not need to declare or redefine it.