The STL standard defines that when an erase occurs on containers such as std::deque, std::list etc iterators are invalidated.
My question is as follows, assuming the list of integers contained in a std::deque, and a pair of indicies indicating a range of elements in the std::deque, what is the correct way to delete all even elements?
So far I have the following, however the problem here is that the assumed end is invalidated after an erase:
#include <cstddef>
#include <deque>
int main()
{
std::deque<int> deq;
for (int i = 0; i < 100; deq.push_back(i++));
// range, 11th to 51st element
std::pair<std::size_t,std::size_t> r(10,50);
std::deque<int>::iterator it = deq.begin() + r.first;
std::deque<int>::iterator end = deq.begin() + r.second;
while (it != end)
{
if (*it % 2 == 0)
{
it = deq.erase(it);
}
else
++it;
}
return 0;
}
Examining how std::remove_if is implemented, it seems there is a very costly copy/shift down process going on.
Is there a more efficient way of achieving the above without all the copy/shifts
In general is deleting/erasing an element more expensive than swapping it with the next nth value in the sequence (where n is the number of elements deleted/removed so far)
Note: Answers should assume the sequence size is quite large, +1mil elements and that on average 1/3 of elements would be up for erasure.