# Thoughts on “sanely move-assignable”

The “relocate” operation to be proposed in my upcoming P1144 (not yet published) is a high-level operation consisting of a move-construct and a destroy. This is analogous to how most libraries today define “swap” as a high-level operation consisting of a move-construct, two move-assigns, and a destroy.

Exchanges values stored in two locations.

Typically, library vendors implement swap as

template<class T>
// requires MoveConstructible<T>
// requires MoveAssignable<T>
// requires Destructible<T> (implied by MoveConstructible<T>)
void swap(T& a, T& b) noexcept(OMITTED)
{
T tmp(std::move(a));
a = std::move(b);
b = std::move(tmp);
}


Mingxin Wang has observed that “swap” can also be expressed in terms of “relocate”. That is, library vendors could reasonably implement a faster std::swap using

template<class T>
// requires MoveConstructible<T>
// requires MoveAssignable<T>
// requires Destructible<T> (implied by MoveConstructible<T>)
void swap(T& a, T& b) noexcept(OMITTED)
{
alignas(T) char buffer[sizeof (T)];
__uninitialized_relocate(&b, &b + 1, (T*)buffer);
__uninitialized_relocate(&a, &a + 1, &b);
__uninitialized_relocate((T*)buffer, (T*)buffer + 1, &a);
}

template<class T>
// requires MoveConstructible<T>
// requires Destructible<T> (implied by MoveConstructible<T>)
void __uninitialized_relocate(T *first, T *last, T *d_first) noexcept(OMITTED)
{
std::uninitialized_move(first, last, d_first);
std::destroy(first, last);
}


(This is an suboptimally generic definition of __uninitialized_relocate, but it suffices in this context.)

Godbolt shows that for std::string, the relocate-based swap (46 instructions on GCC, 50 on Clang) is vastly more efficient than the standard assignment-based swap (174 on GCC, 156 on Clang). It is even more efficient than libstdc++’s insanely complicated string::swap method (78 instructions on GCC; 76 on Clang) — but less efficient than libc++’s (14 instructions on Clang).

This raises a thought-provoking question: If reimplementing swap<T> in this super-efficient way actually stops calling T::operator=, isn’t that an observable side-effect? Imagine this type:

struct Evil {
int i;
Evil(Evil&&) = default;
~Evil() = default;
Evil& operator=(Evil&& rhs) {
i = rhs.i + 1;
puts("I see you");
return *this;
}
};


Notice that Evil is trivially relocatable; but we might say that in some sense it is not “trivially swappable.”

The standard already seems to assume that the visible side-effects of “one move-construction, two move-assignments, and one destruction” must not significantly differ from the visible side-effects of “three move-constructions and three destructions,” or else it would have specified swap’s effects actually in terms of “one move-construction, two move-assignments, and one destruction,” rather than in the vague “Exchanges values” way that it actually did.

However, some paranoid programmer might want to ask, “Is the standard’s assumption actually true of my type T?” That is, “Is my T relocatable and also does move-assigning dst from src perform essentially the same operation as destroying dst and then move-constructing dst from src?” (In the Evil case above, it does not, for at least two reasons.)

The part of the question about the behavior of move-assignment might be separated out and given a name such as is_sanely_move_assignable, at which point we could write the definition

template<class T>
struct is_trivially_swappable : bool_constant<
is_trivially_relocatable_v<T> and
is_sanely_move_assignable_v<T>
> {};


But!

In a C++2a Concepts world, it is extremely reasonable to claim that “is_sanely_move_assignable” essentially encapsulates the semantic requirements of the MoveAssignable syntactic concept. In Concepts, since semantic requirements cannot be indicated in source code, the library must assume that any MoveAssignable type also follows MoveAssignable’s semantic requirements. C++2a Concepts essentially assumes that when I write the syntax x = std::move(y), I get the semantics of a move-assignment; when I write the syntax x + y, I get the semantics of an addition; when I write the syntax *x, I get the semantics of a pointer; and so on.

In short, in a Concepts world, the library must assume that “is_sanely_move_assignable<T> if-and-only-if is_move_assignable<T>.” So, in a Concepts world, there is no need for sanely variants; every operation must be assumed sane by definition.

Therefore, if syntactic-Concepts-with-semantic-requirements remains largely unchanged in C++2a, we safely conclude that

template<class T>
struct is_trivially_swappable : bool_constant<
is_trivially_relocatable_v<T> and
is_move_assignable_v<T>
> {};


Notice that if is_trivially_swappable_v<T>, then swap will check, but never use, the fact that is_move_assignable<T>. Perhaps MoveAssignable should be dropped from the requirements of the swap template?

No, it should not!

One particular kind of type is often trivially relocatable but not move-assignable. In the standard, string_view is assignable and optional<T&> doesn’t exist, but you can imagine versions of both of those where operator= was deleted (to avert misuse, or simply to avert bikeshedding over what it should do). If the programmer has already gone out of his way to disable a = std::move(b), a library’s heroic efforts to reenable std::swap(a, b) are unlikely to be appreciated.

Posted 2018-07-06