Map::operator[] always requires a default constructor

Question

#include <iostream>
#include <string>
#include <map>
using namespace std;

class x{
  public:
  string s;
  //x(){cout<<"default"<<endl;}
  x(string s=""):s(s){cout<<"mine"<<endl;}
};

int main()
{
  map<int,x> m;
  m.insert(pair<int,x>(1,x("me")));
  cout<<m[1].s<<endl;
}

I understand that map::operator[] requires default cst for when element is not present, but

m[1] gives error without default cst for class x in apple clang, even though the element for m[1] is already present. WHY?

Answer 1

I understand that map::operator[] requires default cst for when element is not present, [...]

That's not quite correct.

map::operator[] always requires a default constructor because the element might not be present. Whether it actually is present is not that relevant, because whether a map<T> has a operator[] is decided at compile time already.