Adding types to the std namespace

Question

Is it acceptable to add types to the std namespace. For example, I want a TCHAR-friendly string, so is the following acceptable?

#include <string>

namespace std
{
    typedef basic_string<TCHAR> tstring;
}

Or should I use my own namespace?

Answer 1

Only specializations are allowed. So for example, you are allowed to specialize std::numeric_limits for your type. And this of course must happen in namespace std::. But your typedef isn't a specialization so that's causing undefined behavior.

Answer 2

No ... part of the point of a namespace is to prevent name collisions on upgrade.

If you add things to the std namespace, then your code might break with the next release of the library if they decide to add something with the same name.

Answer 3

[C++11: 17.6.4.2.1/1]: The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

Answer 4

You should use your own namespace as adding code to the standard library will only confuse the users that will look online for informations about that addition.

All that is in std should be only the standard library and nothing else.

Answer 5

Officially, the standard says that's "undefined behaviour", and all kinds of nasty things can happen.

In practice, it will work fine, but you still shouldn't do it. What does it buy you, other than confusing people that something is provided by the compiler?

Answer 6

This is an interesting question because it's completely subjective to the project and the engineers' accepted coding standards.

For a single programmer, why not... just be careful.

For teams, make a standard...

For a cross-platform project, hell yeah.

Otherwise, nawdawg.

Answer 7

I totally agree with other answers saying that you should put your types in your own namespace to avoid unfortunate name collisions.

However, I wanted to precise that sometimes, you can (and should !) add stuff in the std namespace. This is the case for template specializations of the std::swap method for example, which are used to provide a uniform way to swap objects. For more information on this matter, you can read about the non-throwing swap idiom.