C++: Static class members awes

>> I read the following sentence from a c++ website, but can not
>> understand why. can anyone help me with it?
>>
>> "
>> An important detail to keep in mind when debugging or implementing a
>> program using a static class member is that you cannot initialize the
>> static class member inside of the class. In fact, if you decide to put
>> your code in a header file, you cannot even initialize the static
>> variable inside of the header file; do it in a .cpp file instead.
>>
>> "
>
> The Standard requires that every static data member is defined at the
> namespace level. In order to comply with the One Definition rule, you
> are more likely to succeed if you place the definition of the static
> data member in a .cpp file (instead of a header which can be included
> in more than one translation unit). Initialisation accompanies the
> definition. That's why you should initialise static data members in
> a .cpp file (and only in one .cpp file).

Initialization accompanies definition of a constant except in the case when a declaration of a static constant in a class supplies the initializer, in which case the definition (if any, and then outside the class) is sans initializer.

Example:

struct S
{
static int const x = 42; // Not a definition, per §9.4.2/2.
};

If the address of S::x is used, or in the standard's terminology, if S::x is "used", a definition is formally (but with current compilers not necessarily in practice) required, outside the class:

int const S::x; // A definition, per §9.4.2/4.

This construct is only supported for integral types and/including enum types.