Defining static members in C++

Question

I am trying to define a public static variable like this :

public :
         static int j=0;        //or any other value too

I am getting a compilation error on this very line : ISO C++ forbids in-class initialization of non-const static member `j'.

1. Why is it not allowed in C++ ?

2. Why are const members allowed to be initialized ?

3. Does this mean static variables in C++ are not initialized with 0 as in C?

Thanks !

Answer 1

(1.) Why is it not allowed in C++ ?

From Bjarne Stroustrup's C++ Style and Technique FAQ:

A class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects.

(2.) Why are const members allowed to be initialized ?

[dirkgently said it better]

(3.) Does this mean static variables in C++ are not initialized with 0 as in C?

As far as I know, as long as you declare the static member var in a .cpp it will be zero-initialized if you don't specify otherwise:

// in some .cpp
int Test::j; // j = int();