Overly interoperable libraries, part 2

I’ve received a few proposed solutions re “Overly interoperable libraries” (2021-05-19). It’s become clear that I set the initial goalposts a little too close to the kicker. :)

My post asked for a solution to “the general problem,” but provided only a relatively trivial example with two client libraries:

  • Foolib provides class Foo, and wants operator^ to exist whenever the left-hand operand is Foo and the right-hand operand U defines U::foolib_compatible.

  • Barlib provides class Bar, and wants operator^ to exist whenever the right-hand operand is Bar and the left-hand operand T defines T::barlib_compatible.

Easy goal number 1

Peter Dimov and Piotr Nycz presented solutions based on the fact that my sample Foolib cared mainly about the left-hand operand, and my sample Barlib cared mainly about the right-hand operand (Godbolt):

template<class T, class U>
struct enable_xor : std::disjunction<enable_xor_lhs<T, U>,
                                     enable_xor_rhs<T, U>> {};

~~~

template<class U>  // Foo can be xor'ed with anything foolib-compatible.
struct enable_xor_lhs<Foolib::Foo, U, std::enable_if_t<U::foolib_compatible>>
    : std::true_type {};

~~~

template<class T>  // Bar can be xor'ed with anything barlib-compatible.
struct enable_xor_rhs<T, Barlib::Bar, std::enable_if_t<T::barlib_compatible>>
    : std::true_type {};

This approach immediately breaks down if both Foolib and Barlib care about the left-hand operand; for example if we rewrite the requirements to (Godbolt):

  • Foolib provides class Foo, and wants operator^ to exist whenever the left-hand operand is derived from Foo and the right-hand operand U defines U::foolib_compatible.

  • Barlib provides class Bar, and wants operator^ to exist whenever the left-hand operand is derived from Bar and the right-hand operand U defines U::barlib_compatible.

Thus:

struct X : Foolib::Foo, Barlib::Bar { int value() const; } x;
int main() {
    return (x ^ x);  // Collision again!
}

Easy goal number 2: The orphan rule

Chase Albert and Jonathan Müller pointed out that this problem is related to what Rust calls the “orphan rule.” Our “operator^-enablement” is what Rust would call a “trait” — a common behavior that types can opt-into. I don’t know much Rust, but as I understand it, the essence of the orphan rule is that only two people have the authority to “opt a type in” to a trait: the type author, and the trait author. Other components should butt out.

This is a great principle for practical code — so great that my given example adhered to it without consciously intending to. Foolib’s author opted-into operator^ only when at least one of the operands was in fact a type associated with namespace Foolib. Barlib’s author opted-into operator^ only when at least one of the operands was in fact a type associated with namespace Barlib.

C++’s argument-dependent lookup (ADL) takes the same approach: an unqualified fight(you, him) will look up functions named fight in namespaces associated with the types of you and/or him, but a completely unrelated namespace wimpy cannot butt in with its own suggestion.

Jonathan Müller worked up a more or less complete solution based on this approach — basically, recapitulating the ADL associated-namespace rules in partial-specialization form. I’ve massaged it a bit for presentation here. (Godbolt.)

template<class T> constexpr auto libs_of = tl<T>{};
template<class T> constexpr auto libs_of<const T> = libs_of<T>;
template<class T> constexpr auto libs_of<volatile T> = libs_of<T>;
template<class T> constexpr auto libs_of<T*> = libs_of<T>;
template<class T> constexpr auto libs_of<T&> = libs_of<T>;
template<class T> constexpr auto libs_of<T&&> = libs_of<T>;
template<template<class...> class C, class... As> constexpr auto libs_of<C<As...>> = (tl<>{} + ... + libs_of<As>);
template<class R, class... As> constexpr auto libs_of<R(As...)> = (libs_of<R> + ... + libs_of<As>);

template<class T, class U, class LibList>
struct enable_if_xor_is_enabled;
template<class T, class U, class... Libs>
struct enable_if_xor_is_enabled<T, U, tl<Libs...>>
    : std::enable_if<(enable_xor<Libs, T, U>::value || ...), int> {};

template<class T, class U,
    typename enable_if_xor_is_enabled<T, U, decltype(libs_of<T> + libs_of<U>)>::type = 0>
int operator^(T t, U u) {
    return t.value() ^ u.value();
}

~~~

template<class U>  // Foo can be xor'ed with anything foolib-compatible.
struct enable_xor<Foolib::Foo, Foolib::Foo, U, std::enable_if_t<U::foolib_compatible>>
    : std::true_type {};

Unfortunately this approach cannot handle the “derived from Foo” example above, because the generic libs_of has no way to enumerate the public base classes of X and discover that it should be looking for enable_xor specializations associated with Foo and Bar. In other words, our libs_of metaprogramming cannot ever fully recapitulate the rules of ADL.

Alternatively, we could require each type X to specialize libs_of<X> and explicitly name the libraries with which it wants to be associated. But I claim that that idea (1) doesn’t scale, and (2) doesn’t satisfy the problem statement. (We said Foolib wants operator^ to exist whenever the left-hand operand is derived from Foo. We didn’t say “whenever the left-hand operand is derived from Foo and also specializes libs_of.”)

Analogy to smart pointers

The intent of my metaprogramming puzzle is essentially to find a solution that implements “shared ownership” of operator^, in a sense that is exactly analogous to shared_ptr’s “shared ownership” of a heap allocation. We have arbitrarily many entities here, each of which has an equal stake in the existence of operator^; just as with shared_ptr we have arbitrarily many entities, each with an equal stake in the existence of some heap-allocated object.

Rust’s “orphan rule” is analogous to replacing shared_ptr with unique_ptr. The orphan rule highlights that truly “shared” ownership is rarely necessary. We can usually identify a single privileged “owner” for our heap-allocation, and then we can use unique_ptr instead of shared_ptr, and then our code will be easier to reason about.

This is absolutely true! Especially in production code! And usually it turns out to be pretty easy to refactor shared ownership into unique ownership, as Jonathan did in his solution.

However, for the purposes of this puzzle, that’s “cheating.” Even if the libs_of approach were able to fully recapitulate all of C++’s “associated namespace” rules, it still wouldn’t be able to deal with a namespace wimpy that wants to enable operator^ for types not concretely associated with it, e.g.

  • Wimpylib wants operator^ to exist whenever either the left-hand operand T defines T::wimpy_compatible or the right-hand operand U defines U::wimpy_compatible (or both).

My GUID-based toy solution handles this case fine (Godbolt):

template<class T, class U, std::enable_if_t<T::wimpy_compatible, int> = 0>
int enable_xor(priority_tag<22>);  // Wimpy needs two GUIDs; one is "22"

template<class T, class U, std::enable_if_t<U::wimpy_compatible, int> = 0>
int enable_xor(priority_tag<23>);  // and the other is "23"

Perf improvements to the GUID solution

Jonathan Müller offered an alternative GUID solution: Use C++17 fold-expressions to “iterate instead of recurse.” Instead of (Godbolt)

template<class, class>
void enable_xor(priority_tag<0>);

template<class T, class U,
         decltype(enable_xor<T, U>(priority_tag<99>{})) = 0>
int operator^(T t, U u) {
    return t.value() ^ u.value();
}

we can use a fold-expression (Godbolt):

template<size_t K> struct index_tag {};
template<class, class> void enable_xor(index_tag<0>) = delete;

template<size_t I, class T, class U, class = void>
constexpr bool has_enabler = false;
template<size_t I, class T, class U>
constexpr bool has_enabler<I, T, U, decltype(enable_xor<T,U>(index_tag<I>()), void())> = true;

template<class T, class U, size_t... Is>
constexpr bool should_enable_xor(std::index_sequence<Is...>) {
    return (has_enabler<Is, T, U> || ...);
};

template<class T, class U,
         std::enable_if_t<should_enable_xor<T, U>(std::make_index_sequence<99>()), int> = 0>
int operator^(T t, U u) {
    return t.value() ^ u.value();
}

Confusingly, both libc++ and libstdc++ fail to handle this exact solution when index_tag is rewritten as (Godbolt)

template<size_t K>
using index_tag = std::integral_constant<size_t, K>;

I don’t know what’s going wrong there. MSVC (and/or MSVC STL) handles it just fine.

Switching to fold-expressions is unfortunately not a strict win, because of Clang. Clang’s implementation-defined limit on the size of a fold-expression is far less than its implementation-defined limit on recursive template instantiations. GCC and ICC, meanwhile, seem not to limit the size of fold-expressions.

But this reminded me of another classic technique for implementing conjunction and disjunction: Use is_same to do the heavy lifting. To test whether Bs... are all true, we simply compare the type int(bool_constant<Bs>...) against the type int(bool_constant<true>...). Now we have no recursive templates and no large expressions; we simply have a type with a large parameter list. (Godbolt.)

template<bool... Bs>
using And = std::is_same<int(index_tag<Bs>...),
                         int(index_tag<Bs||true>...)>;

template<class T, class U, size_t... Is>
constexpr bool should_enable_xor(std::index_sequence<Is...>) {
    return !And<!has_enabler<Is, T, U> ...>::value;
};

This is far and away the winning strategy on today’s compilers; no compiler bothers to set a detectable limit on “number of function parameters in a function type.”

Technique GCC Clang MSVC ICC
priority_tag 900 1025 497 500
index_tag + fold 5000+ 256 597 33000+
index_tag + same 20000+ 50000+ 35000+ 30000+

So now we’ve got a GUID-based solution permitting four-digit GUIDs. Can we do any better?


See also:

Overly interoperable libraries, part 2