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

The Necessity of Defining Static Data Members Externally to Classes

In the realm of C , a static data member resides within a class but transcends its boundaries, requiring definition outside the class itself. As affirmed by IBM's C knowledge center, "The declaration of a static data member in the member list of a class is not a definition... [it] must define the static member outside of the class declaration, in namespace scope." Why is this rule enforced? What underlying memory allocation considerations necessitate this practice?

The precept behind this mandate lies in the language's fundamental "One Definition Rule." This rule dictates that all static objects employed within a program must bear precisely one definition and only one. Class definitions, typically housed in header files, are frequently incorporated across multiple translation units (source files). Were the static object's declaration placed within the header, each included unit would result in a separate definition, directly contravening the One Definition Rule.

To circumvent this hazard, class definitions within headers merely declare static data members without defining them. This forces the programmer to provide an explicit definition elsewhere. While it is theoretically possible for the language to consolidate multiple definitions into one, as it does with inline functions, this is not the case with static data members. Thus, we are left with the language's requirement for external definition of static data members.

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