Question

So we have set<string> a and set<string> b and we want to get std::set<string> c which would contain items that would represent a - b (meaning what is left from a if we remove from it all items from b, if b contains more than a or items not present in a we want to keep them alike such simple math with numbers: 5-6 = 0 while 3-2 = 1)

Answer 1

I think you want std::set_difference() from <algorithm>.

#include <iostream>
#include <algorithm>
#include <set>
#include <string>
#include <iterator>

using namespace std;

set<string> a;
set<string> b;
set<string> result;


int main()
{
    a.insert("one");
    a.insert("two");
    a.insert("three");

    b.insert("a");
    b.insert("b");
    b.insert("three");

    set_difference( a.begin(), a.end(), b.begin(), b.end(), inserter(result, result.begin()));

    cout << "Difference" << endl << "-------------" << endl;

    for (set<string>::const_iterator i = result.begin(); i != result.end(); ++i) {
        cout << *i << endl;
    }

    result.clear();
    set_symmetric_difference(a.begin(), a.end(), b.begin(), b.end(), inserter(result, result.begin()));

    cout << "Symmetric Difference" << endl << "-------------" << endl;

    for (set<string>::const_iterator i = result.begin(); i != result.end(); ++i) {
        cout << *i << endl;
    }

    return 0;
}