The compiler can use argument-dependent name lookup to find the definition of an unqualified function call. Argument-dependent name lookup is also called Koenig lookup. The type of every argument in a function call is defined within a hierarchy of namespaces, classes, structures, unions, or templates. When you specify an unqualified postfix function call, the compiler searches for the function definition in the hierarchy associated with each argument type.

In the sample, the compiler notes that function f() takes an argument x. Argument x is of type A::X, which is defined in namespace A. The compiler searches namespace A and finds a definition for function f() that takes an argument of type A::X.

// argument_dependent_name_koenig_lookup_on_functions.cpp  
namespace A  
{  
   struct X  
   {  
   };  
   void f(const X&)  
   {  
   }  
}  
int main()  
{  
// The compiler finds A::f() in namespace A, which is where   
// the type of argument x is defined. The type of x is A::X.  
   A::X x;  
   f(x);     
}