Unnamed / anonymous namespaces

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An unnamed namespace can be used to ensure names have internal linkage (can only be referred to by the current translation unit). Such a namespace is defined in the same way as any other namespace, but without the name:

namespace {
    int foo = 42;

foo is only visible in the translation unit in which it appears.

It is recommended to never use unnamed namespaces in header files as this gives a version of the content for every translation unit it is included in. This is especially important if you define non-const globals.

// foo.h
namespace {
    std::string globalString;

// 1.cpp
#include "foo.h" //< Generates unnamed_namespace{1.cpp}::globalString ...

globalString = "Initialize";

// 2.cpp
#include "foo.h" //< Generates unnamed_namespace{2.cpp}::globalString ...

std::cout << globalString; //< Will always print the empty string

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