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.

Answer 2

Section [9.4.2]

Static Data Members

The declaration of a static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void. The definition for a static data member shall appear in a namespace scope enclosing the member’s class definition. In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the :: operator