That's a very optimistic error message! error: conversion from std::_Rb_tree_const_iterator<std::pair< const std::basic_string<char, std::char_traits<char>

That's a very optimistic error message! error: conversion from std::_Rb_tree_const_iterator<std::pair< const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > to non-scalar type std::_Rb_tree_iterator<std::pair< const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > requested