On Wed, Oct 29, 2025 at 3:27 PM Sebastian Wittmeier via Std-Proposals
<std-proposals@lists.isocpp.org> wrote:
>
> sort was just an example.
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> Should generic algorithms prefer swap, when possible?
If it makes sense. The two aren't in competition. One exchanges two
objects that already exist; the other translocates an object to a
different piece of storage, creating a new object in the process.
They're pretty different operations.
"Swap" and "relocate" are two different semantic operations, but it is true that "swap" can be implemented trivially (as-if by swapping bytewise) precisely when "relocate" can be implemented trivially (as-if by relocating bytewise).
One of the killer apps for trivial relocation (although not for the P2786 version that got into C++26, sadly) is that when a type is trivially relocatable, it is also trivially swappable and thus trivially rotate-able, and (as Sean Parent once said) lots of things are rotates.
You can see this live in my libc++ fork:
struct T {
std::shared_ptr<int> a_;
std::unique_ptr<int> b_;
};
#ifdef __cpp_lib_trivially_relocatable
static_assert(std::is_trivially_relocatable_v<T>);
#endif
void f(T *p, int n) {
std::rotate(p, p+1, p+n);
}
Notice the efficient codegen for `rotate` when `T` is understood to be trivially relocatable.
–Arthur