What\'s the Key Difference Between Static Global Variables and Static Data Members in C Header Files?

Distinguishing Static Global Variables from Static Data Members in Header Files

In C , static variables declared in header files exhibit significant differences from those defined within classes.

Static Global Variables in Header Files

Contrary to popular belief, static variables in header files do not possess their own scope. They are included in each source file that references the header, leading to the creation of multiple instances, each associated with the respective translation unit. Consequently, their scope is limited to the translation unit where they are used.

Static Data Members in Classes

In contrast, static data members declared within classes are shared among all instances of the class. This property stems from the concept of internal linkage. Every object of the class accesses the same value for the static data member. The initialization of static data members typically occurs in the .cpp file containing the class definition.

Key Distinction

The distinction lies in the fact that static in a header file context indicates internal linkage, whereas static within a class indicates a shared member variable for all instances of the class.

Implications

Declaring a static variable in a header file is often undesirable, as it can lead to multiple instances with conflicting values. It is preferable to use anonymous namespaces to achieve internal linkage in C instead of relying on static global variables in header files.

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