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2951. iterator_traits should SFINAE for void* and function pointers

Section: 25.3.2.3 [iterator.traits] Status: Resolved Submitter: Billy Robert O'Neal III Opened: 2017-03-24 Last modified: 2020-09-06

Priority: 3

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Discussion:

A customer recently triggered an unexpected SFINAE condition with class path. We constrain the constructor that takes const Source& by asking if iterator_traits<Source>::value_type is one that's OK. This forms iterator_traits<void*>, which is a hard error, as it tries to form void&.

Pointers-to-non-object-type can never act as iterators, as they don't support pointer arithmetic (as we recently discovered in atomic<void*> / atomic<int(*)(int)>).

[2017-07 Toronto Wed Issue Prioritization]

Priority 3; Billy to look into arrays of unknown bounds

[2019-02; Kona Wednesday night issue processing]

This was resolved by the adoption of P0896 in San Diego.

Proposed resolution:

This wording is relative to N4659.

1. Change 25.3.2.3 [iterator.traits] as indicated:

   -3- It is specialized for pointers as

   namespace std {
     template<class T> struct iterator_traits<T*> {
       using difference_type = ptrdiff_t;
       using value_type = T;
       using pointer = T*;
       using reference = T&;
       using iterator_category = random_access_iterator_tag;
     };
   }
   

   and for pointers to const as

   namespace std {
     template<class T> struct iterator_traits<const T*> {
       using difference_type = ptrdiff_t;
       using value_type = T;
       using pointer = const T*;
       using reference = const T&;
       using iterator_category = random_access_iterator_tag;
     };
   }
   

   -?- These partial specializations for pointers apply only when T is an object type.