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Linkage

Example Files
Linkage Linkage.cpp, External.h/.cpp, Internal.cpp
sharing/Sharing sharing/Sharing.cpp and the three constants variants
OneDefinitionRule OneDefinitionRule.cpp - the ODR, inline and templates (draft)
Modules Modules.cpp - C++20 export module / import (draft)

1. Translation units and the linker

Each .cpp file is compiled on its own into an object file (a translation unit). The linker then combines the object files and connects every use of a name with its definition.

Linkage Visible Typical entities
none only in its block local variables
internal only in its .cpp file static globals, const/constexpr globals, anything in an anonymous namespace
external in the whole program non-static functions and globals, extern variables, inline variables

One Definition Rule (ODR): an entity with external linkage must have exactly one definition in the whole program. Inline functions and variables are the exception: they may be defined in several files if every definition is identical.

Common linker errors:

  • undefined reference to ...: something is declared but never defined, or its internal linkage hides it from other files.
  • multiple definition of ...: a non-inline variable or function is defined in a header that is included by several .cpp files.

2. Sharing global constants

Approach Copies Constant expression elsewhere Recompile on change
inline constexpr in a header (C++17) one yes all includers
constexpr in a header one per file yes all includers
extern const + definition in .cpp one no only that .cpp

The Sharing example prints the address of each constant as seen from two different files. This shows that internal constants are copied into every file, while inline constants are shared.