Vector intersection in C++

Instead of sorting, consider trading memory for time by making a hash set out of the smaller vector, and then looping over the larger vector checking for those elements, as suggested here. That would be faster than sorting and using std::set_intersection.

Try std::set_intersection, for example:

#include <algorithm> //std::sort
#include <iostream> //std::cout
#include <string> //std::string
#include <vector> //std::vector

std::vector<std::string> intersection(std::vector<std::string> v1,
                                      std::vector<std::string> v2){
    std::vector<std::string> v3;

    std::sort(v1.begin(), v1.end());
    std::sort(v2.begin(), v2.end());

    std::set_intersection(v1.begin(),v1.end(),
                          v2.begin(),v2.end(),
                          back_inserter(v3));
    return v3;
}

int main(){
    std::vector<std::string> v1 {"a","b","c"};
    std::vector<std::string> v2 {"b","c"};

    auto v3 = intersection(v1, v2);

    for(std::string n : v3)
        std::cout << n << ' ';
}

You need to sort just the smaller vector. Then do a single pass over the bigger vector and test a presence of its items in a smaller vector by using a binary search.