GreenTree GreenTree - 1 year ago 132
C++ Question

no type named ‘pointer’ in struct std::iterator_traits<...>

Here are the fragments of my linked_list template :

#include <iostream>
#include <iterator>

template <class>
struct linked_list {
struct iterator_base : public std::iterator< std::bidirectional_iterator_tag , int >
typename std::iterator_traits< iterator_base >::pointer operator -> () const;
using difference_type = typename std::iterator_traits< iterator_base >::difference_type ;

} ;

linked_list< int > list;

When I leave
line uncommented, the code doesn't compile.

g++5.4 :

list2.cxx:105:66: error: no type named ‘pointer’ in ‘struct std::iterator_traits<linked_list<int>::iterator_base<(linked_list<int, std::allocator<int> >::constantness)1u> >’
typename std::iterator_traits< iterator_base >::pointer operator -> () const { return &( to_obj_node( current_node_ ) -> object() ) ; }

icpc :

list.cxx(105): error: incomplete type is not allowed typename std::iterator_traits< iterator_base >::pointer operator -> () const { return &( to_obj_node( current_node_ ) -> object() ) ; }

Without that line all compiles fine.

the question is : What happens, when i'm commenting
using difference_type = typename std::iterator_traits< iterator >::difference_type;
in above code ( only with such changes code compiles ).?

Answer Source

The error here is that the type std::iterator_traits< iterator_base > is still incomplete and at the time that you want to access ::pointer does not yet provide that pointer member.

The class std::iterator_traits<iterator_base> is being instantiated by typename std::iterator_traits< iterator_base >::difference_type, because it's used on the left side of a ::, and because it has not yet been instantiated. This triggers the instantiation of linked_list<int>::iterator_base because the body of iterator_traits uses that class to define its various member typedefs - for example the instantiation could happen at a line that looks like typedef typename Iterator::value_type value_type; in the Standard library.

What follows is the use of std::iterator_traits< iterator_base >::pointer in your nested class. This time, iterator_traits<iterator_base> is already being instantiated, so nothing is done, and ::pointer is searched. But since that wasn't declared yet, it can't found.

Note that when you comment-out the using line, nothing in the code will instantiate the nested class body anymore (the body of members of class templates are "lazily instantiated"), so this can't be a measure for or against the validity of the constructs inside of that nested class body.

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