Is a Moved-From `std::vector` Always Empty?

Is a Moved-From Vector Always Empty?

Moving objects in C leaves them in an unspecified state. While there is no explicit exclusion for vectors in this rule, finding a feasible implementation that wouldn't result in an empty vector can be challenging.

Move Constructor

The move constructor for vectors must have constant complexity. This necessitates stealing resources from the moved-from vector, leaving it empty. Therefore, for the move constructor, the moved-from vector is always empty.

Move Assignment Operator

The behavior of the move assignment operator depends on the vector's allocator. There are three main cases:

Case 1:
Propagate_on_container_move_assignment is true.

In this case, destruction, deallocation, allocator movement, and memory buffer transfer occur, leaving the moved-from vector empty.

Case 2:
Propagate_on_container_move_assignment is false, but allocators are equal.

Similar to Case 1, the moved-from vector is emptied.

Case 3:
Propagate_on_container_move_assignment is false, and allocators are unequal.

It's impossible to transfer resources directly. Instead, elements must be moved individually. If the moved-from vector contains elements without move constructors, it will remain non-empty. However, the implementation could optionally call v.clear() to empty it, although this is not required by the standard.

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