Quick A: clear() to make sure and use it as normal.
Recently on SO:
From section 17.3.26 of the spec "valid but unspecified state":
an object state that is not specified except that the object’s invariants are met and operations on the object behave as specified for its type [ Example: If an object x of type std::vector<int> is in a valid but unspecified state, x.empty() can be called unconditionally, and x.front() can be called only if x.empty() returns false. —end example ]
Therefore, the object is live. You can perform any operation that does not require a precondition (unless you verify the precondition first).
clear, for example, has no preconditions. And it will return the object to a known state. So just clear it and use it as normal.