What is back_inserter and what is it used for?

std::back_inserter is an inserter iterator from the C++ standard library.

It allows inserting elements at the end of a sequence container (such as std::vector, std::list, std::deque) using algorithms that typically require iterators for output (e.g., std::copy, std::transform).

When using std::back_inserter, the assignment operator (*it = value) calls the container’s push_back method.

The advantage of std::back_inserter is that it automatically manages memory allocation, expanding the container as needed, unlike a regular iterator which requires the target memory area to already be of sufficient size.

A simple example using std::copy:

#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>

int main() {
    std::vector<int> source = {1, 2, 3};
    std::vector<int> destination; // Initially empty container

    // Copy elements from source to the end of destination
    std::copy(source.begin(), source.end(), std::back_inserter(destination));

    // Now destination contains {1, 2, 3}
    for (int val : destination) {
        std::cout << val << " ";
    }
    std::cout << std::endl;

    return 0;
}