According to the C++23 standard, what is the output of this program?
#include <iostream>
namespace x {
class C {};
void f(const C& i) {
std::cout << "1";
}
}
namespace y {
void f(const x::C& i) {
std::cout << "2";
}
}
int main() {
f(x::C());
}Argument-dependent name lookup, aka Koenig lookup, applies.
1
Since the functions f are declared inside namespaces, and the call f(x::C()) is unqualified (not preceded by x:: or y::), this would normally not compile.
However, due to argument-dependent name lookup a.k.a. "Koenig lookup", the behavior is well-defined.
With argument-dependent name lookup, the namespaces of the arguments to a function is added to the set of namespaces to be searched for that function. Since we're passing an x::C to f, the namespace x is also searched, and the function x::f is found.
When the postfix-expression in a function call (§[expr.call]) is an unqualified-id, and unqualified lookup (§[basic.lookup.unqual]) for the name in the unqualified-id does not find any [declaration] then lookup for the name also includes the result of argument-dependent lookup in a set of associated namespaces that depends on the types of the arguments (...).
f is an unqualified-id, so "other namespaces" may be searched.
For each argument type
Tin the function call, there is a set of zero or more associated entities to be considered. The set of entities is determined entirely by the types of the function arguments (...). The set of entities is determined in the following way:— (...)
— If
Tis a class type (including unions), its associated entities are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. (...)
The relevant associated entity here is the class C itself.
§[basic.lookup.argdep]¶4 concludes:
The associated namespaces for a call are the innermost enclosing non-inline namespaces for its associated entities as well as every element of the inline namespace set (§[namespace.def]) of those namespaces. Argument-dependent lookup finds all declarations of functions and function templates that
— are found by a search of any associated namespace (...)
The associated entity is C, and its innermost enclosing namespace is x, which is searched to find f.