How Do C 17 Inline Variables Solve Multiple Definition Problems in Header Files?

Demystifying Inline Variables in C 17

Inline variables, introduced in C 17, empower programmers to define external linkage variables in header files without triggering linker errors.

Mechanism and Functionality:

Similar to inline functions, inline variables can be defined in a header file with identical definitions in multiple translation units. Thanks to the extended machinery that supports static variables in class templates, the compiler can intelligently handle these multiple definitions.

Declaration and Usage:

To declare an inline variable:

• Use the inline keyword to specify external linkage
• Choose a constant namespace-scope variable or a static class data member

Utility and Examples:

Inline variables offer several advantages:

• Code Simplification: They eliminate the need for complex tricks like template tricks to achieve similar functionality.
• Code Reuse: Reusable global constants can now be easily defined and used from multiple translation units.
• Simplified Initialization: Static data members with brace-or-equal-initializers can be defined in the class definition and redeclared in namespace scope without an initializer.

Example:

struct Kath
{
    static inline std::string const hi = "Zzzzz...";
};

This declaration allows you to access the constant Kath::hi from any translation unit that includes the header file.

Additional Enhancements:

• The constexpr specifier implies inline for static data members.
• Inline variables can have external linkage, but only one definition will be used by the linker.

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