Question

 //foo.h
 class Foo 
  {
    private:
      static int number;

    public: 
      static int bar();
  };

//foo.cc
#include "foo.h"

 int Foo::bar() 
 {
   return Foo::number;
 }

this is not working. I want to define a static function outside the class definition and access a static value.

undefined reference to `Foo::number'

Answer 1

You just declared the static member you need to define it too.
Add this in your cpp file.

int Foo::number = 0;

This should be a good read:

what is the difference between a definition and a declaration?