Question

Consider the following code:

std::vector vec;
vec.reserve(500);
size_t cap = vec.capacity();

std::vector newVec = std::move(vec);
assert(cap == newVec.capacity());

In pretty much any implementation you run across, this will work. I don’t care about what implementations do. I want to know what the standard requires. Will the moved-to vector have the same capacity as the original? Or will the assert trigger?

Answer 1

Looking at the standard, it appears that nothing is required from the move constructor, however as @amaurea says, it would completely defeat the purpose of move semantics if the move constructor were to try and allocate or deallocate memory, so I would expect the capacity to remain the same in all implementations.

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23.2.1 General container requirements

Expression

X u(a);
X u = a;

Assertion/note pre-/post-condition

Requires: T is CopyInsertable into X (see below).
post: u == a

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The standard only requires that newVec == vec. As capacity is not taken into consideration for std::vector::operator==, newVec need not necessarily have the same capacity as vec.