Just wanted to add why this idiom is used at all, which seemed like the spirit of the original question. This idiom is used within many std library classes where swap is implemented. From http://www.cplusplus.com/reference/algorithm/swap/: Many components of the standard library (within std) call swap in an unqualified manner to allow custom overloads for non-fundamental … Read more
This mechanism is normally used in templated code, i.e. template <typename Value> class Foo. Now the question is which swap to use. std::swap<Value> will work, but it might not be ideal. There’s a good chance that there’s a better overload of swap for type Value, but in which namespace would that be? It’s almost certainly … Read more
X::A a; f(a); works because of Argument-Dependent Lookup (Also known as Koenig Lookup). a is an object of class A inside namespace X, when compiler searches a match-able function f, it will look into namespace X in this case. See Argument Dependent Lookup for more information.
There is a huge problem with argument-dependent lookup. Consider, for example, the following utility: #include <iostream> namespace utility { template <typename T> void print(T x) { std::cout << x << std::endl; } template <typename T> void print_n(T x, unsigned n) { for (unsigned i = 0; i < n; ++i) print(x); } } It’s simple … Read more
This part explains it: C++ Standard 03 14.8.1.6: [Note: For simple function names, argument dependent lookup (3.4.2) applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call (3.4.1). But when a function template with explicit template … Read more
Koenig Lookup, or Argument Dependent Lookup, describes how unqualified names are looked up by the compiler in C++. The C++11 standard ยง 3.4.2/1 states: When the postfix-expression in a function call (5.2.2) is an unqualified-id, other namespaces not considered during the usual unqualified lookup (3.4.1) may be searched, and in those namespaces, namespace-scope friend function … Read more