Convert iterator to string ref Problem

When containers such as map are used to convert the iterator into a string object, an error is returned:

Error c2440: 'initializing': cannot convert from 'const class STD: basic_string <char, struct STD: char_traits <char>, class STD :: allocator <char> 'to' class STD: basic_string <char, struct STD: char_traits <char>, class STD: Allocator <char>
> &'

As you can see, in a map container similar to the following definition

Map <string, int> mapstringkey;

The key (string) is internally converted to const string for storage.

String & strkey = itr-> first;

An error is reported,

The Correct assignment method is as follows:

1, string strkey = itr-> first;

2, const string & strkey = itr-> first;

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We can see from the following article that the key type defined by map is of the const type.

template < class Key, class T, class Compare = less<Key>,           class Allocator = allocator<pair<const Key,T> > > class map;

Http://www.cplusplus.com/reference/stl/map/