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

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!