Question

In C and C++ all static variables are initialized by default to ZERO.

This is not the case of static class data members. Why is that?

#include <iostream>
using namespace std;

int var;

class MyClass
{
public:
    static int classVar;
};
int MyClass::classVar = 0;  // Why I have to init it here?

int main(void)
{
    cout << ::var << endl;          // this is initalized to ZERO by default
    static int var;
    cout << var << endl;            // and this also is initalized to Zero
    cout << MyClass::classVar << endl;

    return 0;
}

Answer 1

At class scope,

int MyClass::classVar = 0;  // Why I have to init it here?

is a definition and

static int classVar;

is a declaration, ie. a promise the variable will be defined somewhere: you must define exactly once the variables you declare.

The rationale is that the class declaration will likely be included in multiple source files. Would a part of it be a definition, it would take place multiply: this is erroneous (exceptions are inline [member] functions).

Note that according to value initialization rules, you can get along with

int MyClass::classVar;  // Zero-initialized !

as a definition.

Variables declared at namespace scope are definitions too (unless they are extern qualified):

int var;

is a declaration, and a definition: if you put this into a header and include it in multiple translation units, you have an error (“multiply defined symbol”, or something along those lines).

[Note that in C++ (and not in C), if the var above is const, it becomes automatically static and there is no violation of the One Definition Rule should it be put into a multiply included header. This goes slightly off topic, but feel free to ask details]