error c2675 using STL VS13 c++ - c++

Question

I am keeping reading Stanford tutorials and currently solving tasks on STL. The task is to write a function which accepts a map with names of films and their rankings. This function should return a set container with top 3 films according to critics review. Here is my solution:
#include <iostream>
#include <set>
#include <map>
#include <numeric>
#include <iterator>
#include <string>
using namespace std;
struct compare {
bool operator() (const double& a, const double& b) const {
return (a > b);
}
};
set <string> list(map <double, string, compare>& films) {
set <string> critics;
map<double, string, compare> ::iterator it = films.begin();
if (films.size() <= 3) {
critics.insert(films.begin()->second, films.end()->second);
}
else {
for (int i = 0; i < 3; ++i, ++it){
critics.insert(it->second);
}
};
return critics;
}
int main() {
map <double, string, compare> films;
films[5.0] = "a";
films[8.0] = "b";
films[10.0] = "c";
films[7.4] = "d";
set <string> critics = list(films);
copy(critics.begin(), critics.end(), ostream_iterator <string>(cout, " "));
cin.get();
}
Unfortunately, it keeps on throwing out an error:
error C2675: unary '++' : 'std::string' does not define this operator or a conversion to a type acceptable to the predefined operator
I have read MSDN documentation on the error but as I am new to this could not understand the meaning of the problem. Could you, please, hint me at that?

Answer 1

This statement
critics.insert(films.begin()->second, films.end()->second);
is invalid. The compiler consideres arguments films.begin()->second and films.end()->second of type std::string as a pair of iterators and tries to apply operator ++. Of course this results in an error.
You should use standard algorithm std::transform with std::insert_iterator that to copy strings from the map to the set.
Here is a demonstrative program that shows the approach
#include <iostream>
#include <map>
#include <set>
#include <string>
#include <algorithm>
#include <iterator>
#include <functional>
int main()
{
std::map<double, std::string, std::greater<double>> m =
{
{ 2.2, "B" }, { 1.1, "A" }, { 4.4, "D" }, { 5.5, "E" }, { 3.3, "C" }
};
for ( const auto &p : m )
{
std::cout << p.first << '\t' << p.second << std::endl;
}
std::set<std::string> s;
std::transform( m.begin(), std::next( m.begin(), 3 ),
std::inserter( s, s.end() ),
[]( const auto &p ) { return p.second; } );
for ( const auto &t : s ) std::cout << t << ' ';
std::cout << std::endl;
return 0;
}
The program output is
5.5 E
4.4 D
3.3 C
2.2 B
1.1 A
C D E