In a C++ program, each global symbol can be defined only once, even if the program consists of multiple translation units. A translation unit consists of an implementation file (.cpp, .hpp, .cxx, etc) and all the headers that it includes directly or indirectly. A program consists of one or more translation units linked together.

The concept of linkage refers to the visibility of global symbols (such as variables, type names and function names) within the program as a whole across translation units. The concept of scope refers to symbols that are declared within a block such as a namespace, class, or function body. Such symbols are visible only within the scope in which they are defined; the concept of linkage does not apply to them.

Non-const global variables and free functions by default have external linkage; they are visible from any translation unit in the program. You can override this behavior by explicity declaring them as static which limits their visiblity to the same translation unit in which they are declared. This meaning of static is different than its meaning when applied to local variables. Variables declared as const have internal linkage by default.

In earlier versions of Visual Studio, the compiler always gave a constexpr variable internal linkage even when the variable was marked extern. In Visual Studio 2017 version 15.5, a new compiler switch (/Zc:externConstexpr) enables correct standards-conforming behavior. Eventually this will become the default.

extern constexpr int x = 10; //error LNK2005: "int const x" already defined

If a header file contains a variable declared extern constexpr, it needs to be marked __declspec(selectany) in order to correctly have its duplicate declarations combined:

extern constexpr __declspec(selectany) int x = 10;

Basic Concepts