Why Initialize C Static Member Variables Outside the Class Declaration?

Initialization of Static Member Variables in C : Why Outside the Class?

In C , static member variables are initialized outside the class declaration. This deviation from the more intuitive in-class initialization has sparked curiosity among developers. Is there a logical reason behind this design decision?

The fundamental reason lies in the requirement for static members to be defined in exactly one translation unit. If initialization were allowed within the class, it could lead to multiple definitions of the same variable in different translation units, violating the One-Definition Rule.

To illustrate this, consider the following example:

struct Gizmo {
  static string name = "Foo";
};

If this code were allowed, name would be defined in each translation unit that includes this header file. This conflicts with the One-Definition Rule, which mandates that a variable can only have one definition.

However, C provides a syntactic sugar that allows the initialization of integral static members within the declaration:

struct Gizmo {
  static const int count = 42;
};

This is permissible as long as the expression is a const integral or enumeration type, can be evaluated at compile-time, and there is still a definition in a single translation unit that does not violate the One-Definition Rule. For instance:

// gizmo.cpp
#include "gizmo.h"

const int Gizmo::count;

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