Does a vector assignment invalidate the `reserve`?

Question

Suppose I write

std::vector littleVector(1);
std::vector bigVector;

bigVector.reserve(100);
bigVector = littleVector;

Does t

Answer 1

The only requirement on operator= for standard containers is that afterwards, src == dst, as specified in Table 96 (in 23.2, General Container Requirements). Furthermore, the same table specifies the meaning of operator ==:

distance(lhs.begin(), lhs.end()) == distance(rhs.begin(), rhs.end()) // same size
  && equal(lhs.begin(), lhs.end(), rhs.begin()) // element-wise equivalent

Note that this doesn't include capacity in any way. Nor does any other part of the standard mention capacity beyond the general invariant that capacity() >= size(). The value of capacity after assignment is therefore unspecified, and the container is free to implement assignment whichever way it wants, as long as allocator requirements are kept.

---

In general, you will find that implementations behave such that

• if the allocators compare equal and dst has sufficient capacity, it will retain its old storage,
• otherwise it will allocate just enough storage for the new elements, and
• in no case will care what the capacity of src is.

Of course, move assignment is a different story. Since it is generally implemented by stealing the source storage, the capacity will be taken as well.