Question

say I have vector<class1a>,vector<class1b> how to remove the common entities from both of them I have defined ==operator for the class1 objects class1a,class1b

Answer 1

The stl algorithms provide several functions to perform set operations, notably calculating the set symmetric difference, which is what you need.

Here’s an example of use:

#include <algorithm> #include <vector>  int main(int argc, char **argv) {      std::vector<int> v1;     v1.push_back(1);     v1.push_back(2);     v1.push_back(3);     v1.push_back(4);     v1.push_back(5);     v1.push_back(6);      std::vector<int> v2;     v2.push_back(2);     v2.push_back(4);     v2.push_back(6);     v2.push_back(8);      // Ranges must be sorted!     std::sort(v1.begin(), v1.end());     std::sort(v2.begin(), v2.end());      std::vector<int> res; // Will contain the symmetric difference     std::set_symmetric_difference(v1.begin(), v1.end(),                                    v2.begin(), v2.end(),                                    std::back_inserter(res));      // Copy result to the output     std::copy(res.begin(), res.end(), std::ostream_iterator<int>(cout, ' '));     // Prints '1 3 5'      return 0; } 

std::set_symmetric_difference takes two range (i.e. two pairs of OutputIterators) and an InputIterator where it will put the result. It also returns an iterator to the end of the result range.

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EDIT

I just read your comments on your question. If you want the two original vectors to be modified, you can use std::set_difference:

vector<int>::iterator endRange; endRange = set_difference(v1.begin(), v1.end(),                            v2.begin(), v2.end(),                            v1.begin()); v1.erase(endRange, v1.end()); 

Here, we put the result of the set difference v1 – v2 into v1. However, we can’t do the vice-versa since v1 is now modified. The solution is to calculate the intersection of v1 and v2, and then the difference with this intersection std::set_intersection :

vector<int> inter; set_intersection(v1.begin(), v1.end(),                  v2.begin(), v2.end(),                  back_inserter(inter)); // inter is '2 4 6'  v1.erase(set_difference(v1.begin(), v1.end(),                         inter.begin(), inter.end(),                         v1.begin()),          v1.end()); // v1 is '1 3 5'  v2.erase(set_difference(v2.begin(), v2.end(),                         inter.begin(), inter.end(),                         v2.begin()),          v2.end()); // v2 is '8' 

I guess there are much more performant solutions, but this one is clear, and really convey your intents by using widely known stl algorithms.