Why do non-const, non-int/enum static data members have to be initialized outside the definition?

Question

I understand that only data members which are static, const and int/enum (pre c++11) can be initialized inside the class declaration. "All other static data members must be defined at global namespace scope (i.e. outside the body of the class definition) and can be only initialized in those definitions".

Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?

If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?

Answer 1

Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?

Most likely because C++ has separate translation units. The compiler needs to pick an object file where the initialization logic for those symbols will be placed. Forcing this to be in a specific source file makes that decision easy for the compiler.

If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?

Because that's just how C++ does class members. This is no different than other class members like member functions:

Header file:

namespace example {

// Class declared in header
struct some_class
{
    // Member variable
    static float example;
    // Member function
    void DoStuff() const;
};

}

Source file:

namespace example {

    // Implement member variable
    float some_class::example = 3.14159;
    // Implement member function
    void some_class::DoStuff() const
    {
         //....
    }
}

There's a specific exception to allow static const integral members to be initialized in the header because it allows the compiler to treat them as compile-time constants. That is, you can use them to define sizes of arrays or other similar bits in the class definition.

Answer 2

Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?

In general, all static objects require a definition, in one single translation unit, so that they have a well-defined address. As a special exception, static, constant, non-volatile class members don't need a definition if their address is not required, and they have a simple enough type that their value can be replaced by a compile-time constant.

Historically, "simple enough" was defined as an integral or enumeration type; C++11 extends that to include any literal type with a constexpr specifier.

If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?

They are not declared at the global namespace scope. They are declared and scoped within the class.

If you mean, why are they defined outside the class definition, that's because there must be only one definition of the static member in the whole program; but the class must be defined in each translation unit that uses it.