Can std::vector emplace_back copy construct from an element of the vector itself?

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emplace_back is required to be safe for the same reason push_back is required to be safe; invalidation of pointers and references only has effect once the modifying method call returns.

In practice, this means that emplace_back performing a reallocation is required to proceed in the following order (ignoring error handling):

  1. Allocate new capacity
  2. Emplace-construct new element at the end of the new data segment
  3. Move-construct existing elements into new data segment
  4. Destruct and deallocate old data segment

At this reddit thread STL acknowledges failure of VC11 to support v.emplace_back(v[0]) as a bug, so you should definitely check whether your library supports this usage and not take it for granted.

Note that some forms of self-insertion are specifically prohibited by the Standard; for example in [sequence.reqmts] paragraph 4 Table 100 a.insert(p,i,j) has the prerequisite “i and j are not iterators into a“.

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