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Containers also give more efficient and simpler semantics if move constructors are noexcept, which they generally should be.

std::vector does. No other container cares about noexcept at all.

Ah yes, I forgot that only vector can relocate elements. (Other containers can invalidate iterators but the objects themselves aren't moved.)

They could be even more efficient if we had a way to indicate relocatability instead of having to fall back to more general move semantics.