C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

61. Don’t define entities with linkage in a header file

Summary

Repetition causes bloat: Entities with linkage, including namespace-level variables or functions, have memory allocated for them. Defining such entities in header files results in either link-time errors or memory waste. Put all entities with linkage in implementation files.

Discussion

While starting to use C++, we all learn quite quickly that header files like

are liable to cause link-time errors complaining of duplicate symbols as soon as such a header is included by more than one source file. The reason is simple: Each source file actually defines and allocates space for fudgeFactor ...