Why Do I Get \'Undefined References\' When Using Static Class Members in C ?

Undefined References to Static Class Members

In C , static members defined within classes require proper handling to avoid compilation errors.

Reason for Undefined References

Static members, despite being declared within class definitions, are not automatically defined unless explicitly done elsewhere. This is because declarations differ from definitions in C . A declaration merely introduces the member, while a definition allocates memory and provides an initial value.

Example

Consider the following class with an undeclared static member:

class Example {
    static bool exampleStaticMember;
};

Attempting to use exampleStaticMember without defining it will result in "undefined references" errors.

Definition Requirements

To resolve this issue, the static member must be explicitly defined, typically in the source file (.cpp) that contains the class definitions. The definition simply declares the member again with its data type and a semicolon (';').

bool Example::exampleStaticMember;

Special Cases

• Const Integral/Enumeration Static Members: These can be initialized within the class definition itself.
• Static Template Members: Static members of class templates require definition in the header file.

Other Static Usages

Note that the static keyword has different meanings when applied outside of classes:

• Static Objects in Functions: Declares an object that retains its value between function calls.
• Static Objects/Functions in Namespace Scope: Declares objects/functions with internal linkage.

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