Static Variable initialisation for Classes in C++, why include data type?

Question

I've been learning C++, and I've come across static variable (I have prior knowledge from C89), and in the resource i'm using, they've declared a static variable in a class such as:

class nameHere
{
    public:
        static int totalNum;
}

int nameHere::totalNum = 0;

int main()
{}

For Example. What I don't understand is that, since I've already declared that the static variable is an integer in the class definition, why do I need to also declare it as an integer outside of the class definition? Would it not make sense to simply initialise it like so:

nameHere::totalNum = 0;

int main()
{}

Is there a particular reason or simply a convention of C++? Thanks for all the help!

Answer 1

This would (probably) make the language even more difficult to parse (and it's already almost insanely difficult to parse anyway).

As it is, the datatype (int, long, my_class, whatever) tells the compiler that what it's seeing is the beginning of a declaration (which, in this case, is also a definition). Without that, the compiler would have a rather more difficult time sorting things out.

In the specific case of things at global scope, it wouldn't be that bad, because at global scope about all you can have is a series of declarations. At any other scope, however, things would be more difficult (and having one rule at global scope, and another elsewhere would be ugly indeed).

Answer 2

In C++11 you can simply initialize the variable inside the class:

class nameHere
{
    public:
        static const int totalNum = {0};
}

Answer 3

There is a difference between a definition and a declaration. While the static variable in the class has been declared, it has not been defined. The One Definition Rule, explains declarations and definitions and states

In any translation unit, a template, type, function, or object can have no more than one definition. Some of these can have any number of declarations.

Therefore, the full type of the object must be used when declaring the variable.