Most efficient way of erasing/deleting multiple std::vector elements while retaining original order?

Question

i have a std::vector and a second container holding iterators or indexes (no keys, i want constant access to the element) to this vector for del

Answer 1

If you have a (e.g. unordered) set of indices that you want to erase, you can use this:

template <typename Type>
void erase_indices(
        const std::unordered_set<size_t>& indices_to_erase,
        std::vector<Type>& vec) {
    std::vector<bool> erase_index(vec.size(), false);
    for (const size_t i: indices_to_erase) {
        erase_index[i] = true;
    }
    std::vector<bool>::const_iterator it_to_erase = erase_index.cbegin();
    typename std::vector<Type>::iterator it_erase_from = std::remove_if(
        vec.begin(), vec.end(),
        [&it_to_erase](const Type&) -> bool {
          return *it_to_erase++ == true;
        }
    );
    vec.erase(it_erase_from, vec.end());
}

It is the fastest solution that came to my mind. You need C++11, though. Usage example to erase elements at index 2 and 5:

constexpr size_t num = 10u;
std::vector<int> vec(num);
std::iota(vec.begin(), vec.end(), 0);

std::unordered_set<size_t> indices_to_erase;
indices_to_erase.insert(2u);
indices_to_erase.insert(5u);

erase_indices(indices_to_erase, vec);

Before:

0 1 2 3 4 5 6 7 8 9

After:

0 1 3 4 6 7 8 9

Edit: If want to be more flexible regarding type of container that hold the indices to erase:

template <typename Type, typename Container>
void erase_indices(
        const Container& indices_to_erase,
        std::vector<Type>& vec) {
    typedef typename Container::value_type IndexType;
    static_assert(std::is_same<IndexType, std::size_t>::value,
        "Indices to be erased have to be of type std::size_t");
    std::vector<bool> erase_index(vec.size(), false);
    for (const IndexType idx_erase: indices_to_erase) {
        erase_index[idx_erase] = true;
    }
    std::vector<bool>::const_iterator it_to_erase = erase_index.cbegin();
    typename std::vector<Type>::iterator it_erase_from = std::remove_if(
        vec.begin(), vec.end(),
        [&it_to_erase](const Type&) -> bool {
          return *it_to_erase++ == true;
        }
    );
    vec.erase(it_erase_from, vec.end());
}

Now you can use any kind of container from the Containers Library to provide the indices to be erased as long as the value_type of that container is std::size_t. Usage remains the same.