Ovanes Markarian wrote:
Sorry,
you are right. I forgot that all operators are overloaded or defined in std namespace.
It's a bit more subtle than that. As a general rule, an operator needs to be defined in the namespace of the class for which it applies. If you had std::ostream& operator<<( std::ostream& os, my::X const & x ); you'd need to put this operator in namespace my, because that's where the class my::X resides. All this assumes that your compiler supports argument-dependent lookup (ADL); for older compilers, the rule is "put the operator wherever it appears to work." However, since the operator you are defining: template <typename T> std::ostream& operator<<( std::ostream& os, const std::vector<T>& v ); applies to std::vector, it needs to be defined in namespace std. In principle, a class N::X is supposed to also define the necessary support operators in namespace N, but std::vector doesn't do that (mainly because it isn't clear how is this operator supposed to work; there are many legitimate ways to output a sequence.) This is a general C++ problem, not specific to Boost.Variant, so it's understandable why the documentation doesn't mention it.