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c++unnamed-namespace

Why should types be put in unnamed namespaces?

I understand the use of unnamed namespaces to make functions and variables have internal linkage. Unnamed namespaces are not used in header files; only source files. Types declared in a source file cannot be used outside. So what's the use of putting types in unnamed namespaces?

See these links where it's mentioned that types can be put in unnamed namespaces:

Solution

  • Where do you want to put local types other than the unnamed namespace? Types can't have a linkage specifier like static. If they are not publicly known, e.g., because they are declared in a header, there is a fair chance that names of local types conflict, e.g., when two translation units define types with the same name. In that case you'd end up with an ODR violation. Defining the types inside an unnamed namespace eliminates this possibility.

    To be a bit more concrete. Consider you have

    // file demo.h
int foo();
double bar();

// file foo.cpp
struct helper { int i; };
int foo() { helper h{}; return h.i; }

// file bar.cpp
struct helper { double d; }
double bar() { helper h{}; return h.d; }

// file main.cpp
#include "demo.h"
int main() {
     return foo() + bar();
}

    If you link these three translation units, you have mismatching definitions of helper from foo.cpp and bar.cpp. The compiler/linker is not required to detect these but each type which is used in the program needs to have a consistent definition. Violating this constraints is known as violation of the "one definition rule" (ODR). Any violation of the ODR rule results in undefined behavior.

    Given the comment it seems a bit more convincing is needed. The relevant section of the standard is 3.2 [basic.def.odr] paragraph 6:

    There can be more than one definition of a class type (Clause 9), enumeration type (7.2), inline function with external linkage (7.1.2), class template (Clause 14), non-static function template (14.5.6), static data member of a class template (14.5.1.3), member function of a class template (14.5.1.1), or template specialization for which some template parameters are not specified (14.7, 14.5.5) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then each definition of D shall consist of the same sequence of tokens; and [...]

    There are plenty of further constraints but "shall consist of the same sequence of tokens" is clearly sufficient to rule out e.g. the definitions in the demo above from being legal.