Which namespace does operator<< (stream) go to?

Question

If I have have some overloaded ostream operators, defined for library local objects, is its okay for them to go to std namespace? If I do not declare them in std namespace, then I must use using ns:: operator <<.

As a possible follow-up question, are there any operators which should go to standard or global namespace?

Answer 1

It is generally a bad practice to declare anything (types, operators, etc ...) to be a part of a namespace you do not own. This can have unexpected consequences for people consuming your library. A better solution is to define your own namespace and to import both std and your namespace when you need to combine solutions.

Answer 2

According to Koenig Lookup (C++ Standard 3.4.2) operator<< will be searched in namespaces of arguments. No need to declare it in std namespace.

Answer 3

The C++ Standard explicitly forbids you from declaring your own constructs in the std namespace.

Answer 4

operator<<( ..., MyClass ) should go in the same namespace as MyClass. You should think of it as part of the interface of MyClass, even though it happens to be (necessarily) a non-member function.

A couple of references:

• My article What's In a Class?.
• C++ Coding Standards Item 57: "Keep a type and its nonmember function interface in the same namespace."