C++ Namespaces Argument Dependent Lookup
Example
When calling a function without an explicit namespace qualifier, the compiler can choose to call a function within a namespace if one of the parameter types to that function is also in that namespace. This is called "Argument Dependent Lookup", or ADL:
namespace Test
{
int call(int i);
class SomeClass {...};
int call_too(const SomeClass &data);
}
call(5); //Fails. Not a qualified function name.
Test::SomeClass data;
call_too(data); //Succeeds
call fails because none of its parameter types come from the Test namespace. call_too works because SomeClass is a member of Test and therefore it qualifies for ADL rules.
When does ADL not occur
ADL does not occur if normal unqualified lookup finds a class member, a function that has been declared at block scope, or something that is not of function type. For example:
void foo();
namespace N {
struct X {};
void foo(X ) { std::cout << '1'; }
void qux(X ) { std::cout << '2'; }
}
struct C {
void foo() {}
void bar() {
foo(N::X{}); // error: ADL is disabled and C::foo() takes no arguments
}
};
void bar() {
extern void foo(); // redeclares ::foo
foo(N::X{}); // error: ADL is disabled and ::foo() doesn't take any arguments
}
int qux;
void baz() {
qux(N::X{}); // error: variable declaration disables ADL for "qux"
}
