Why Must Static Data Members Be Defined Outside the Class Declaration in C ?

Memory Allocation Implications of Static Data Members

The IBM C knowledge center states that the declaration of a static data member in the class member list is not a definition, requiring it to be defined outside of the class declaration in namespace scope. This raises the question of why this is necessary and what the schematic implications are regarding memory allocation.

According to the One Definition Rule, each static object within a program must be defined exactly once. If the static object's declaration in the header file were a definition, it would result in multiple definitions across translation units, violating the rule.

To avoid this, the declaration in the header file is not a definition. Instead, a single definition must be provided outside of the class definition, typically in the source file of the primary translation unit. This ensures that the static object is defined only once in the entire program, adhering to the One Definition Rule.

In terms of memory allocation, static data members are allocated in a special segment of memory known as the static data segment. They are initialized only once, during program startup, and retain their values throughout the program's lifetime. This is in contrast to non-static data members, which are allocated in the stack or heap when an object is created and destroyed when the object is destroyed.

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