# Overly interoperable libraries, part 3

Previously on this blog:

Jason Cobb has sent me a proposal that I think meets all the requirements and therefore qualifies as a complete solution!

Unlike the ideas in my previous posts, Jason’s solution doesn’t depend on “GUIDs” at all. It’s based on a simple idea that is, in hindsight, quite obvious.

It’s tricky to distinguish “zero” from “one or two or more.”

But it’s easy to distinguish “exactly one” from “two or more.”

Jason’s solution to the original puzzle looks like this (Godbolt):

struct tag {};
struct disable_tag {};

template<class T, class U>
disable_tag enable_xor(tag);

template<class T, class U, class = disable_tag>
struct is_xor_enabled : std::true_type {};

template<class T, class U>
struct is_xor_enabled<T, U, decltype(enable_xor<T, U>(tag{}))> : std::false_type {};

template<class T, class U, std::enable_if_t<is_xor_enabled<T, U>::value, int> = 0>
int operator^(T t, U u) {
return t.value() ^ u.value();
}

~~~

template<class T, class U,
// Foo can be xor'ed with anything foolib-compatible.
std::enable_if_t<std::is_same<T, Foolib::Foo>::value, int> = 0,
std::enable_if_t<U::foolib_compatible, int> = 0>
void enable_xor(tag);

~~~

template<class T, class U,
// Bar can be xor'ed with anything barlib-compatible.
std::enable_if_t<std::is_same<U, Barlib::Bar>::value, int> = 0,
std::enable_if_t<T::barlib_compatible, int> = 0>
void enable_xor(tag);


The overload set for the decltype’d call to enable_xor<T, U>(tag{}) consists of the initial disable_tag overload, plus any of the library-specific overloads that remain viable after template substitution. If any library cares about this particular set of arguments <T, U>, then the decltype’d call to enable_xor<T, U>(tag{}) will be ambiguous and is_xor_enabled<T, U>::value will be true. If no library cares about <T, U>, then the decltype’d call will not be ambiguous — the only viable candidate will be the initial disable_tag overload — and so is_xor_enabled<T, U>::value will be false.

This solution easily handles my followup post’s Wimpylib example (Godbolt):

// Any wimpy-compatible type can be xored with any other type.
template<class T, class U, std::enable_if_t<T::wimpy_compatible, int> = 0>
void enable_xor(tag);
template<class T, class U, std::enable_if_t<U::wimpy_compatible, int> = 0>
void enable_xor(tag);


Whatever the prize was for solving this puzzle, Jason has won it!

The code above uses struct disable_tag merely to get a unique return type that won’t collide with the return type of any of the user’s library overloads. You could replace it with something like int, no problem.

But struct tag cannot be replaced with int! If you do that, suddenly the compiler stops finding the library overloads of enable_xor as candidates at all. The reason is two-phase lookup. When we ask for decltype(enable_xor<T, U>(tag{})) in a dependent context, the compiler considers all of the candidates found by ordinary unqualified lookup right now (in “phase one”), plus all of the candidates found by ADL at instantiation time (“phase two”). The compiler does not consider candidates found by ordinary unqualified lookup in phase two! So it’s actually super important that the various library overloads be placed in a namespace associated with one of enable_xor’s function arguments.

int is a primitive type: it has no associated namespaces. struct tag, on the other hand, has one associated namespace — the global namespace. So, because we use struct tag as the function argument type in our decltype’d enable_xor<T,U>(tag{}), ADL is able to find all these enable_xor overloads that the various libraries have dumped into the global namespace.

I feel like I’ve seen this “add one” trick somewhere before. I’m not sure if this is exactly where I saw it, but at least it occurs to me that you can use this trick to determine whether a given type has any member named m, even when that member is non-public. Godbolt:

template<class T, class = void>
constexpr bool naive_has_m = false;
template<class T>
constexpr bool naive_has_m<T, std::void_t<typename T::m>> = true;

class YesButPrivate { using m = int; };


naive_has_m<YesButPrivate> gives false, because the name T::m is private. (Except on GCC, where it’s currently a hard error: that’s a bug.) So, add one…

struct Sibling { using m = void; };
template<class T> struct Helper : T, Sibling {};
template<class T, class = void>
constexpr bool smart_has_m = true;
template<class T>
constexpr bool smart_has_m<T, typename Helper<T>::m> = false;


smart_has_m<YesButPrivate> gives true, because name lookup on Helper<T>::m results in ambiguity. Only when T::m does not exist at all are we able to look up typename Helper<T>::m unambiguously. (In that case of course it will be public and void.)

Posted 2021-05-26