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

Static Global Variables and Static Data Members: A Clarification

The distinction between static global variables defined in header files and static data members declared within classes can be confusing. This article aims to shed light on their differences.

Static Global Variables in Header Files

Contrary to popular belief, there is no such concept as "header file scope." When a header file is included in a source file, its contents are essentially copied verbatim into the latter. Therefore, a static global variable declared in a header file exists in every translation unit that includes it.

Unlike the internal linkage associated with static variables in functions or member functions, a static global variable in a header file has external linkage. This means it can be accessed externally to its translation unit, leading to potential conflicts and confusion.

Static Data Members in Classes

In contrast, a static data member declared within a class has different semantics. It is shared among all instances of that class. Even though static data members are initialized in a non-class translation unit (typically a .cpp file), the scope of the data member extends to all instances of the class across the entire program.

Key Differences

• Scope: Static global variables have external linkage and are visible across all translation units, while static data members have class linkage and are shared among instances of the same class.
• Initialization: Static global variables are typically initialized in a translation unit separate from their declaration, while static data members are initialized in the class's non-class translation unit.
• Usage: Static global variables are generally discouraged in C , as they can lead to undesirable interactions between different parts of the codebase. Static data members are a preferred mechanism for declaring class-wide shared data.

Best Practice

As a rule of thumb, using anonymous namespaces is a more robust alternative to static global variables for achieving internal linkage in C .

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