How to efficiently delete elements from a vector given an another vector

Question

What is the best way to delete elements from a vector given an another vector?

I have come up with the following code:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

void remove_elements(vector<int>& vDestination, const vector<int>& vSource) 
{
    if(!vDestination.empty() && !vSource.empty())
    {
        for(auto i: vSource) {
            vDestination.erase(std::remove(vDestination.begin(), vDestination.end(), i), vDestination.end());
        }
    }
}

int main() 
{
    vector<int> v1={1,2,3};
    vector<int> v2={4,5,6};
    vector<int> v3={1,2,3,4,5,6,7,8,9};
    remove_elements(v3,v1);
    remove_elements(v3,v2);
    for(auto i:v3)
        cout << i << endl;
    return 0;
}

Here the output will be:

7
8
9

Answer 1

My version is the following, I only apply erase after all elements from the vector vSource have been moved to the end by std::remove and keep track of the pointer to the end of the vector vDestination to not iterate over it for nothing.

void remove_elements(vector<int>& vDestination, const vector<int>& vSource) 
{
    auto last = std::end(vDestination);
    std::for_each(std::begin(vSource), std::end(vSource), [&](const int & val) {
        last = std::remove(std::begin(vDestination), last, val);
    });
    vDestination.erase(last, std::end(vDestination));
}

See on coliru : http://coliru.stacked-crooked.com/a/6e86893babb6759c

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Update

Here is a template version, so you don't care about the container type :

template <class ContainerA, class ContainerB>
void remove_elements(ContainerA & vDestination, const ContainerB & vSource) 
{
    auto last = std::end(vDestination);
    std::for_each(std::begin(vSource), std::end(vSource), [&](typename ContainerB::const_reference val) {
        last = std::remove(std::begin(vDestination), last, val);
    });
    vDestination.erase(last, std::end(vDestination));
}

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Note

This version works for vectors without any constraints, if your vectors are sorted you can take some shortcuts and avoid iterating over and over the vector to delete each element.