C++: Storing std::set_union() output in a std::multiset

Question

I want to perform a union operation between two multisets .

However, at the moment I have been able to figure out only to store the output of the operation into a std::vector , and then I have to convert a vector to a multiset.

This conversion from my tests is very expensive and I would like to avoid that because I need the output as a multiset.

How can I achieve this?

Code:

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

int main () {
    multiset<int> m, p;
    m.insert(4);
    m.insert(4);
    m.insert(2);
    m.insert(1);
    m.insert(7);

    p.insert(2);
    p.insert(2);
    p.insert(2);
    p.insert(1);
    p.insert(5);

    vector<int> v;
    set_union(m.begin(), m.end(), p.begin(), p.end(), back_inserter(v));

    multiset<int> result(v.begin(), v.end());

    multiset<int>::iterator it;
    for (it=result.begin(); it!=result.end(); it++)
        cout << *it << " ";

    return 0;

}

The output is 1 2 2 2 4 4 5 7 as expected.

Answer 1

You need an std::insert_iterator . It works like std::back_inserter , but just inserts instead of inserting at the back. You can create one by calling std::inserter(c) , where c is the target container. The first example here shows an inserter into an std::multiset .