Why Are My Header Files Still Causing Multiple Definition Errors Despite Using Include Guards?

Why Isn't My Code Protected by Include Guards?

Recursive Inclusion

Include guards are effective at preventing mutual recursive inclusion of header files. When processing #include directives, the preprocessor checks for existing macro definitions before parsing the header's content. If the macro is already defined, it skips the header's content, effectively preventing recursion.

Multiple Definitions

In contrast to mutual inclusion, include guards are not designed to prevent multiple symbol definitions. When a header is included in multiple translation units (separate .cpp files), the symbol definitions are replicated in each unit. This can lead to linker errors due to the violation of the One Definition Rule in C .

Solutions

Recursive Inclusion:
Forward declare classes or data structures in header files to allow for dependencies without mutual inclusion.

Multiple Definitions:

• Inline: Use the inline keyword for functions or global variables defined in header files that are included in multiple translation units. The compiler will attempt to inline the function at its call site, avoiding multiple symbol definitions.
• Internal Linkage: Use static or unnamed namespaces to give symbols internal linkage, limiting their visibility to the current translation unit. This is less desirable than inline as it can increase executable size.

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