如何使用以下模板为地图声明迭代器-std :: map <std::string, T> my_map？

Question

I have the following class declaration -

template <typename T> 
class Polynomial{
  std::map<std::string, T> _polynomial_
}

In a member function I declared an iterator for this -

typename std::map<std::string, T>::iterator it= _polynomial_.begin();

The completed member function looks like this -

template <typename T>
void Polynomial<T>::print(std::ostream& out) const
{


  typename std::map<std::string, T>::iterator it= _polynomial_.begin();
  std::string term;
  while(it != _polynomial_.end()){


    term = it->second;
    term += it->first;
    if(it->first < (T)0){
      out << "-" << term;
    }
    else{
      out << "+" << term;
    }
    term = "";
    it++;
  }


}

In main, I call the function as follows -

 Polynomial <double> p1;

  p1.add_term("x0",9.862);

  std::cout << p1;

However this does not seem to work and I get errors. GCC complains of a conversion error -

Polynomial.hpp:32:47: error: conversion from \‘std::map, double, std::less >, std::allocator, double> > >::const_iterator {aka std::_Rb_tree_const_iterator, double> >}\’ to non-scalar type \‘std::map, double, std::less >, std::allocator, double> > >::iterator {aka std::_Rb_tree_iterator, double> >}\’ requested typename std::map::iterator it= polynomial .begin();

Can someone tell me what is the correct declaration of the iterator?

Answer 1

Polynomial<T>::print is a const member function, inside which the data member _polynomial_ becomes const too, that means what _polynomial_.begin() returns is a const_iterator , which can't be converted to iterator implicitly. (Note that std::map::begin is overloaded with const version and non- const version, the former returns const_iterator and the latter returns iterator .)

Change the code to

typename std::map<std::string, T>::const_iterator it = _polynomial_.begin();
//                                 ^^^^^^

or use auto instead, it would deduce the correct type for you.

auto it = _polynomial_.begin();