Functions with class arguments are leaked from a namespace?

Question

I have a small piece of code here for your consideration which puzzles me quite a lot. The strange thing is that it compiles on both Sun Studio and GCC even though I think it sh

Answer 1

This is Argument-Dependent Name Lookup, a.k.a. ADL, a.k.a. Koenig lookup. This was invented to make operators work as expected, e.g.:

namespace fu {
    struct bar { int i; };
    inline std::ostream& operator<<( std::ostream& o, const bar& b ) {
        return o << "fu::bar " << b.i;
    }
}

fu::bar b;
b.i = 42;
std::cout << b << std::endl; // works via ADL magic

Without ADL you'd have to either explicitly bring the output operator with ugly using fu::operator<<;, or use even uglier explicit call:

fu::operator<<( std::cout, b ) << std::endl;