Does moving leave the object in a usable state?

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From n3290, 17.6.5.15 Moved-from state of library types [lib.types.movedfrom]

  1. Objects of types defined in the C++ standard library may be moved from (12.8). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.

Since the state is valid, this means you can safely operate on v2 (e.g. by assigning to it, which would put it back to a known state). Since it is unspecified however, it means you cannot for instance rely on any particular value for v2.empty() as long as it is in this state (but calling it won’t crash the program).

Note that this axiom of move semantics (“Moved from objects are left in a valid but unspecified state”) is something that all code should strive towards (most of the time), not just the Standard Library components. Much like the semantics of copy constructors should be making a copy, but are not enforced to.

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