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

Perplexed by the External Definition of Static Data Members? Unraveling the Enigma

According to IBM's C knowledge center, it is imperative to define static data members outside the class declaration. This raises the question: why is this necessary, and what are the underlying memory allocation principles dictating this rule?

To address this, we must delve into the concept known as the One Definition Rule. In C , each static object within a program must be defined precisely once and only once. Class definitions are often included in header files, which are in turn incorporated into multiple translation units.

If the static object's declaration within the header were also its definition, multiple definitions would arise with each inclusion of the header file. This contravenes the One Definition Rule, leading to potential breakage.

Therefore, the static object is not defined within the header. Instead, an external, singular definition is provided elsewhere in the codebase.

Theoretically, the language could emulate the handling of inline functions, permitting multiple definitions to be merged into one. However, the C language does not adopt this approach, necessitating compliance with the external definition rule for static data members.

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