static int counter // will initalized to 0

but if I make that variable inside a class, it's not initialized and I have to initialize it outside of class

class Test {
static int counter // not initialized
}
...
Test::counter = 0; 

I know the static variables are stored in BSS segment in memory and initialized by default to 0, so why is it when I make a static in class not initialized?

CodePudding user response：

Why static global variables initialized to zero, but static member variable in class not initialized?

Because the declaration for a non-inline static data member inside the class is not a definition.

This can be seen from static data member documentation:

The static keyword is only used with the declaration of a static member, inside the class definition, but not with the definition of that static member. The declaration inside the class body is not a definition and may declare the member to be of incomplete type (other than void), including the type in which the member is declared:

Also the out of class definition Test::counter = 0; is incorrect. It should instead be int Test::counter = 0;