Why is the static variable initialized with redeclaration of static variable outside the class? Can't we just initialize it instead of redeclaring it?

Question

#include <iostream>

using namespace std;

class Box
{
   public:
      static int objectCount;
}

// Initialize static member of class Box
int Box::objectCount = 0;

Primitive types like `int` actually _can be_ initialized at the point of declaration of `static` class members. — πάντα ῥεῖ, Feb 07 '15 at 12:15
That was an omission on the part of earlier C++ standard, which has been fixed in a later edition of the standard. — Sergey Kalinichenko, Feb 07 '15 at 12:19
Huh? To counter the earlier comments, GCC rejects in-class initialisation with "error: ISO C++ forbids in-class initialization of non-const static member", and clang does with "error: non-const static data member must be initialized out of line". Both regardless of `-std=*` options. Which standard supposedly allows this? — , Feb 07 '15 at 12:29

score 3 · Accepted Answer · answered Feb 07 '15 at 12:24

It seems that you mix up the declaration and the definition of a variable.

The declaration just tells the compiler a name. So in your case:

class Box
{
    public:
        static int objectCount;
};

This just tells the compiler that there is a variable with the name objectCount.
But now you still need a definition.

int Box::objectCount = 0;

Simplified the definition is what the linker needs.
So as a simple rule static member variables must be declared in the class and then defined outside of it.

Why is the static variable initialized with redeclaration of static variable outside the class? Can't we just initialize it instead of redeclaring it?

1 Answers1