c++ - Is `using namespace foo;` useless?

Suppose I have a library that places all its declarations in namespace foo. I can then conveniently use the library by adding:

using namespace foo;

to my source files. However, if any innocent name in foo (for example class foo::socket) is now ambiguous with global namespace names (for example ::socket()). What's more, there is no way to disambiguate them! Looks like using namespace is only useful for libraries that don't conflict with the global namespace, or am I missing something?

EDIT: by "disambiguate" I meant once for the scope, not at every use site, which is of course possible.

I guess we'll have to wait for modules for a satisfactory solution.