Question

Why last line doesnt work?

ostringstream stream;
int f = 12345;
stream << f;
string st = stream.str();
cout << typeid(st.rbegin()).name() << endl;
cout << typeid(stream.str().rbegin()).name() << endl;
cout << string(st.rbegin(), st.rend()) << std::endl;
cout << string(stream.str().rbegin(), stream.str().rend()) << endl;

ir says: “String iterators incompatible”, but types the same.

Answer 1

Calling the ostringstream::str() method creates a new string object, therefore you’re using iterators from two different objects.

To solve the issue, store the string in a temporary variable:

string temp = stream.str();
cout << string(temp.rbegin(), temp.rend()) << endl;

Which is what you were doing with the st variable already.