const global variable in namespace

Question

I know that static const class members can be initialised in headers only. Is it the same for namespaces ? For example, is it valid to write :

namehuman.hpp

namespace namehuman
{
   string const human("human");
}

main.cpp

#include "namehuman.hpp"
cout << namehuman::human << endl;

I am wondering if all files including the header file will have their own copy of string human, or if human will be a true global variable (not copied many times). In order to avoid each including file making its copy, am I obliged to use extern ?

Answer 1

Constants have internal linkage. Thus any compilation unit that includes the header with the definition of the constant will have its own instance of the object.

According to the C++ Standard (3.5 Program and linkage)

3 A name having namespace scope (3.3.6) has internal linkage if it is the name of

...

— a non-volatile variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage; or

If you want a constant with external linkage, you must declare it with the specifier extern, and define it in a compilation unit.

Answer 2

I think that will define the human several times, so it can happen that this causes an ODR violation (see below). It's usually best to only declare it in the header

extern const string human;

and add the definition to the implementation file

string human("human");

Be careful about the initialization order fiasco and the equivalent when closing the application.

An ODR violation can be caused when an inline function with external linkage ODR-uses human. I think that, since that is really easy to do and there is no way to guard against it, it is best to define constant strings in the implementation file.