c++vectorreferencelanguage-lawyerpush-back

Is it safe to push_back an element from the same vector?

vector<int> v;
v.push_back(1);
v.push_back(v[0]);

If the second push_back causes a reallocation, the reference to the first integer in the vector will no longer be valid. So this isn't safe?

vector<int> v;
v.push_back(1);
v.reserve(v.size() + 1);
v.push_back(v[0]);

This makes it safe?

Solution

  • It looks like http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#526 addressed this problem (or something very similar to it) as a potential defect in the standard:

    1) Parameters taken by const reference can be changed during execution of the function

    Examples:

    Given std::vector v:

    v.insert(v.begin(), v[2]);

    v[2] can be changed by moving elements of vector

    The proposed resolution was that this was not a defect:

    vector::insert(iter, value) is required to work because the standard doesn't give permission for it not to work.