c++ - How to access static members of a class?

Question

I am starting to learn C++ and Qt, but sometimes the simplest code that I paste from a book results in errors.

I'm using g++4.4.2 on Ubuntu 10.04 with QtCreator IDE. Is there a difference between the g++ compiler syntax and other compilers? For example when I try to access static members something always goes wrong.

#include <iostream>
using namespace std;
class A
{
   public:
      static int x;
      static int getX() {return x;}
};
int main()
{
   int A::x = 100; // error: invalid use of qualified-name 'A::x'
   cout<<A::getX(); // error: : undefined reference to 'A::x'
   return 0;
}

I think it's exactly the same as declared here and here (isn't it?). So what's wrong with the above code?

Answer 1

You've declared the static members fine, but not defined them anywhere.

Basically what you've said "there exists some static member", but never set aside some memory for it, you need:

int A::x = 100;

Somewhere outside the class and not inside main.