set_union() is not working for a set of strings

I was trying to find out the union of two set containing strings, using set_union(...) function. But, it's throwing error inside of stl_algo.h ar line no 4948 -

Error : passing 'const std::__cxx11::basic_string<char>' as 'this' argument discards qualifiers [-fpermissive]

My code :

#include<bits/stdc++.h>

using namespace std;

int main()
{
    int t,k, tmp, i=1,j,l,m,n,x1,x2;
    cin>>n;
    string st,stt;
    set <string> set1,set2,set3;
    set1.insert("sdsd");
    set1.insert("sdswewd");
    set1.insert("ssd");

    set2.insert("sdsd");
    set2.insert("sdfewew");
    set2.insert("ssd");
    set_union(set1.begin(),set1.end(),set2.begin(),set2.end(),set3.begin());

    return 0;
}

try using std::inserter

set_union( set1.begin(), set1.end(), set2.begin(), set2.end(),std::inserter( set3, set3.begin() ));

Update:

a1.begin() is simply not an output iterator. inserter(a1,a1.begin()) returns an output iterator which will invoke the set's insert function for each element... Why do we need an inserter function call when doing a set_union for a set?

Also since we are dealing with std::set a container where uniqueness is guaranteed we don't need to take a set_union because a simple set insert will also ensure it that no copies of same element is created.

//insert all element of set 1 to set 3
set3.insert(set1.begin(),set1.end());
//insert all elements of set 2 (that is not in set 1) to set 3
set3.insert(set2.begin(),set2.end());