ADL using std types fails to find operator

Question

The following code fails to compile

namespace A {
using C = std::vector;
std::ostream& operator << (std::ostream& lhs, cons         


        

Answer 1

A::C is just a type alias, and aliases are transparent. They don't "remember" where they came from. When we do argument-dependent lookup and figure out what the associated namespaces are, we only consider the associated namespaces of the types - not the alias that got us there. You can't just add associated namespaces to existing types. The specific associated namespace of f (which is of type std::vector<std::string>) is std, which doesn't have an operator<< associated with it. Since there's no operator<< found using ordinary lookup, nor is there one found using ADL, the call fails.

Now, I said you can't just add associated namespaces to existing types. But you can of course just create new types:

namespace A {
    struct C : std::vector<std::string> { };
}

or:

namespace A {
    // template parameters are also considered for associated namespaces
    struct S : std::string { };
    using C = std::vector<S>;
}