Why will two-phase lookup fail to choose overloaded version of 'swap'?

Question

I am studying this fascinating answer to a subtle question regarding the best practice to implement the swap function for user-defined types.

Answer 1

It is impossible to overload swap in namespace std for a user defined type. Introduction an overload (as opposed to a specialization) in namespace std is undefined behavior (illegal under the standard, no diagnosis required).

It is impossible to specialize a function in general for a template class (as opposed to a template class instance -- ie, std::vector<int> is an instance, while std::vector<T> is the entire template class). What appears to be a specialization is actually an overload. So the first paragraph applies.

The best practice for implementing user-defined swap is to introduce a swap function or overload in the same namespace as your template or class lives in.

Then, if swap is called in the right context (using std::swap; swap(a,b);), which is how it is called in std library, ADL will kick in, and your overload will be found.

The other option is to do a full specialization of swap in std for your particular type. This is impossible (or impractical) for template classes, as you need to specialize for each and every instance of your template class that exists. For other classes, it is fragile, as specialization applies to only that particular type: subclasses will have to be respecialized in std as well.

In general, specialization of functions is extremely fragile, and you are better off introducing overrides. As you cannot introduce overrides into std, the only place they will be reliably found from is in your own namespace. Such overrides in your own namespace are preferred over overrides in std as well.

There are two ways to inject a swap into your namespace. Both work for this purpose:

namespace test {
  struct A {};
  struct B {};
  void swap(A&, A&) { std::cout << "swap(A&,A&)\n"; }
  struct C {
    friend void swap(C&, C&) { std::cout << "swap(C&, C&)\n"; }
  };

  void bob() {
    using std::swap;
    test::A a, b;
    swap(a,b);
    test::B x, y;
    swap(x, y);
    C u, v;
    swap(u, v);
  }
}

void foo() {
  using std::swap;
  test::A a, b;
  swap(a,b);
  test::B x, y;
  swap(x, y);
  test::C u, v;
  swap(u, v);

  test::bob();
}
int main() {
  foo();
  return 0;
}

the first is to inject it into the namespace directly, the second is to include it as an inline friend. The inline friend for "external operators" is a common pattern that basically means you can only find swap via ADL, but in this particular context does not add much.

Answer 2

Preamble with plenty of Standardese

The call to swap() in the example entails a dependent name because its arguments begin[0] and begin[1] depend on the template parameter T of the surrounding algorithm() function template. Two-phase name lookup for such dependent names is defined in the Standard as follows:

14.6.4.2 Candidate functions [temp.dep.candidate]

1 For a function call where the postfix-expression is a dependent name, the candidate functions are found using the usual lookup rules (3.4.1, 3.4.2) except that:

— For the part of the lookup using unqualified name lookup (3.4.1), only function declarations from the template definition context are found.

— For the part of the lookup using associated namespaces (3.4.2), only function declarations found in either the template definition context or the template instantiation context are found.

Unqualified lookup is defined by

3.4.1 Unqualified name lookup [basic.lookup.unqual]

1 In all the cases listed in 3.4.1, the scopes are searched for a declaration in the order listed in each of the respective categories; name lookup ends as soon as a declaration is found for the name. If no declaration is found, the program is ill-formed.

and argument-dependent lookup (ADL) as

3.4.2 Argument-dependent name lookup [basic.lookup.argdep]

1 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 or function template declarations (11.3) not otherwise visible may be found. These modifications to the search depend on the types of the arguments (and for template template arguments, the namespace of the template argument).

Applying the Standard to the example

The first example calls exp::swap(). This is not a dependent name and does not require two-phase name lookup. Because the call to swap is qualified, ordinary lookup takes place which finds only the generic swap(T&, T&) function template.

The second example (what @HowardHinnant calls "the modern solution") calls swap() and also has an overload swap(A&, A&) in the same namespace as where class A lives (the global namespace in this case). Because the call to swap is unqualified, both ordinary lookup and ADL take place at the point of definition (again only finding the generic swap(T&, T&)) but another ADL takes place at the point of instantiation (i.e where exp::algorithm() is being called in main()) and this picks up swap(A&, A&) which is a better match during overload resolution.

So far so good. Now for the encore: the third example calls swap() and has a specialization template<> swap(A&, A&) inside namespace exp. The lookup is the same as in the second example, but now ADL does not pick up the template specialization because it is not in an associated namespace of class A. However, even though the specialization template<> swap(A&, A&) does not play a role during overload resolution, it is still instantiated at the point of use.

Finally, the fourth example calls swap() and has an overload template<class T> swap(A<T>&, A<T>&) inside namespace exp for template<class T> class A living in the global namespace. The lookup is the same as in the third example, and again ADL does not pick up the overload swap(A<T>&, A<T>&) because it is not in an associated namespace of the class template A<T>. And in this case, there is also no specialization that has to be instantiated at the point of use, so the generic swap(T&, T&) is being callled here.

Conclusion

Even though you are not allowed to add new overloads to namespace std, and only explicit specializations, it would not even work because of the various intricacies of two-phase name lookup.