Why Must Static Member Variable Initialization in C Occur Outside the Class?

Static Member Variables in C : Initialization Outside the Class

While it may seem counterintuitive, the initialization of static member variables in C occurs outside the class for several logical reasons.

One-Definition Rule Compliance

Static members must be defined in exactly one translation unit, adhering to the One-Definition Rule. If initialization were allowed within the class, it would lead to multiple definitions of the same variable in different translation units, violating this rule.

Example:

Consider the following code snippet:

struct Gizmo {
  static string name = "Foo"; // Not allowed
};

If this initialization were permitted, the variable name would be defined in every translation unit that includes the header file, violating the One-Definition Rule.

Allowing In-Class Initialization

While allowing initialization within the class may appear more intuitive, it would still require a separate definition to ensure that the One-Definition Rule is enforced. Thus, allowing in-class initialization would only add syntax without providing any real benefit.

Alternative for Integral Values

For integral static members, C allows initialization within the declaration if the expression is:

• Const integral or enumeration type
• Evaluable at compile-time

This allows for a syntactic shortcut that makes it easier to define integral static members with a single line of code. However, a definition in a separate translation unit is still required.

Example:

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

In this case, the compiler generates the necessary definition outside the class to comply with the One-Definition Rule.

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