What is the correct way to reuse a moved container?
std::vector<int> container;
container.push_back(1);
auto container2 = std::move(container);
// ver1: Do nothing
//container2.clear(); // ver2: "Reset"
container = std::vector<int>() // ver3: Reinitialize
container.push_back(2);
assert(container.size() == 1 && container.front() == 2);
From what I've read in the C++0x standard draft; ver3 seems to be the correct way, since an object after move is in a
"Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state."
I have never found any instance where it is "otherwise specified".
Although I find ver3 a bit roundabout and would have much preferred ver1, though vec3 can allow some additional optimization, but on the other hand can easily lead to mistakes.
Is my assumption correct?
§17.3.26 valid but unspecified state [defns.valid]
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
xof typestd::vector<int>is in a valid but unspecified state,x.empty()can be called unconditionally, andx.front()can be called only ifx.empty()returnsfalse. — 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.