What is the most effective way to move items within a vector?

Question

I\'ve seen some special cases where std::rotate could be used or a combination with one of the search algorithms but generally: when one has a vector of N items

Answer 1

Pre-C++11 (although the following remains valid) you can get more efficient "moves" for contained types which specialise/overload std::swap. To take advantage of this, you would need to do something like

std::vector new_vec;
Foo tmp;

for (/* each Foo&f in old_vec, first section */) {
    swap (f, tmp);
    new_vec .push_back (tmp);
}

for (/* each Foo&f in old_vec, second section */) {
    swap (f, tmp);
    new_vec .push_back (tmp);
}

for (/* each Foo&f in old_vec, third section */) {
    swap (f, tmp);
    new_vec .push_back (tmp);
}

swap (new_vec, old_vec);

The above may also give good results for C++11 if Foo has a move-operator but hasn't specialised swap.

Linked lists or some clever sequence type might work out better if Foo doesn't have move semantics or an otherwise-optimised swap

Note also that if the above is in a function

std::vector move (std::vector old_vec, ...)`

then you might be able to perform the whole operation without copying anything, even in C++98 but for this to work you will need to pass by value and not by reference, which goes against the conventional prefer-pass-by-reference wisdom.