Is there a function to remove an element from a vector without shifting it in the c++ stdlib?

Question

If I use vector.erase() such as

std::vector<int> n = { 3, 5, 6, 7 };
n.erase(n.begin() + 1);

vector will shift all the elements after the element removed down.

Is there a function in the C++ standard library that will not shift the elements? Like putting the back element at the removed element and pop back?

Answer 1

There isn't anything built in, but it's trivial to do yourself.

std::vector<int> n = { 3, 5, 6, 7 }; // create vector
n[1] = std::move(n.back());          // move last element to removal location
n.pop_back();                        // remove unneeded element.

Answer 2

If you don't want to shift any elements, then in C++20 you can model this with a vector of std::optional s and the ranges library

#include <cstddef>
#include <optional>
#include <ranges>
#include <vector>

template<typename T>
auto remove_element(std::vector<std::optional<T>> &v, size_t i) {
    v[i] = std::nullopt;

    return v
        | std::views::filter(&std::optional<T>::has_value)
        | std::views::transform([](auto &&o) {return *o;});
}

#include <iostream>

int main() {
    std::vector<std::optional<int>> v{1, 2, 3, 4, 5, 6};

    for (int i : remove_element(v, 3))
        cout << i << ','; // 1,2,3,5,6,

    cout << '\n';
}