cpp17-input-iterator
concept is needlessly complexSection: 24.3.2.3 [iterator.traits] Status: New Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2020-09-06
Priority: 3
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Discussion:
The new C++20 iterator concepts use common_reference
to constrain the value, reference, and
rvalue_reference associated types in order to support proxy references (see
24.3.4.2 [iterator.concept.readable]).
common_reference
in
24.3.2.3 [iterator.traits]/p2 is needlessly complex. The common_reference
constraints can be
replaced with simple convertibility requirements to a const
lvalue reference to the value type.
This fix has been implemented in range-v3.
[2019-10-14 Issue Prioritization]
Priority to 3 after reflector discussion.
Proposed resolution:
This wording is relative to N4830.
Modify 24.3.2.3 [iterator.traits] as indicated:
-2- The definitions in this subclause make use of the following exposition-only concepts:
template<class I> concept cpp17-iterator = copyable<I> && requires(I i) { { *i } -> can-reference; { ++i } -> same_as<I&>; { *i++ } -> can-reference; }; template<class I> concept cpp17-input-iterator = cpp17-iterator<I> && equality_comparable<I> && requires(I i) { typename incrementable_traits<I>::difference_type; typename readable_traits<I>::value_type;typename common_reference_t<iter_reference_t<I>&&, typename readable_traits<I>::value_type&>; typename common_reference_t<decltype(*i++)&&, typename readable_traits<I>::value_type&>;{ *i } -> convertible_to<const typename readable_traits<I>::value_type&>; { *i++ } -> convertible_to<const typename readable_traits<I>::value_type&>; requires signed_integral<typename incrementable_traits<I>::difference_type>; }; […]