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;

Answer 2

It's due to "argument dependent lookup". Removing the const will not change the behavior you're seeing. To demonstrate that it's ADL, try moving the St struct outside of the namespace...

struct St
{
   int a;
};

namespace name
{
  void f(const St& st);
  void g(int a);
}

int main(int argc, char** argv)
{
  St st;

  name::f(st); 
  f(st);  // <--- now you will get the expected compile error

  name::g(42);
  // g(42);  <--- this does not, as I expected
}

Answer 3

That is caused by argument dependent lookup.

Answer 4

It's called argument-dependent lookup (or Koenig lookup). In short, the compiler will look for the function in namespaces that are the namespaces of argument types.