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Question

I am trying to define an iterator that iterates through a puzzle game board cells by a given starting cell and some link representing some kind of a connection between the cells. Below I provided a sample code with the same methods but a trivial implementation that potentially should iterate over the cells with the coordinates {0, 0}, {1, 1}, {2, 2}, {3, 3}, {4, 4}:

#include <ranges>

struct CellCoord { size_t col; size_t row; };

struct NeighborIteratorSentinel {};

class NeighborIterator
{
public:

    NeighborIterator(size_t link_id) :
        linkId(link_id)
    {}

    using iterator_category = std::forward_iterator_tag;

    using value_type = CellCoord;

    using difference_type = std::ptrdiff_t;

    using pointer = value_type*;

    using reference = value_type&;

    CellCoord operator-> () const { return cur(); }

    CellCoord operator* () const { return cur(); }

    NeighborIterator& operator++ ()
    {
        ++linkId;

        return *this;
    }

    NeighborIterator operator++ (int)
    {
        NeighborIterator tmp = *this;

        ++linkId;

        return tmp;
    }

    NeighborIterator(const NeighborIterator& other) = default;
    NeighborIterator(NeighborIterator&& other) = default;

    NeighborIterator& operator = (const NeighborIterator& other) = default;
    NeighborIterator& operator = (NeighborIterator&& other) = default;

    NeighborIterator& operator = (const NeighborIteratorSentinel&)
    {
        linkId = linkCount;
    }

    NeighborIterator& operator = (NeighborIteratorSentinel&&)
    {
        linkId = linkCount;
    }

    bool operator == (const NeighborIterator& r) const noexcept
    {
        return linkId == r.linkId;
    }

    bool operator != (const NeighborIterator& r)  const noexcept
    {
        return !operator == (r);
    }

    bool operator == (const NeighborIteratorSentinel&) const noexcept
    {
        return linkId == linkCount;
    }

    bool operator != (const NeighborIteratorSentinel& r)  const noexcept
    {
        return !operator == (r);
    }

protected:

    CellCoord cur() const
    {
        return {linkId, linkId};
    }

private:

    size_t linkId;

    static constexpr size_t linkCount = 5;
};

The problem is that I can’t compile the following method:

inline auto MakeNeighborRange()
{
    return std::ranges::subrange<NeighborIterator, NeighborIteratorSentinel>(NeighborIterator(0u), NeighborIteratorSentinel{});
}

because std::sentinel_for constraint is not satisfied.

How to make this compile? What does this constraint wand of me?

MSVC output:

C:devtemp>cl /std:c++20 /EHsc NeighborIterator.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.39.33523 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

NeighborIterator.cpp
NeighborIterator.cpp(101): error C7602: 'std::ranges::subrange': the associated constraints are not satisfied
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(3735): note: see declaration of 'std::ranges::subrange'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(3733): note: the concept 'std::sentinel_for<example::NeighborIteratorSentinel,example::NeighborIterator>' evaluated to false
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519include__msvc_iter_core.hpp(421): note: the concept 'std::_Weakly_equality_comparable_with<example::NeighborIteratorSentinel,example::NeighborIterator>' evaluated to false
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(193): note: the concept 'std::_Half_equality_comparable<example::NeighborIteratorSentinel,example::NeighborIterator>' evaluated to false
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(187): note: 'bool example::NeighborIterator::operator ==(const example::NeighborIteratorSentinel &) noexcept const': rewritten candidate function was excluded from overload resolution because a corresponding operator!= declared in the same scope
NeighborIterator.cpp(75): note: could be 'bool example::NeighborIterator::operator ==(const example::NeighborIteratorSentinel &) noexcept const' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(187): note: 'bool example::NeighborIterator::operator ==(const example::NeighborIteratorSentinel &) noexcept const': rewritten candidate function was excluded from overload resolution because a corresponding operator!= declared in the same scope
NeighborIterator.cpp(65): note: or 'bool example::NeighborIterator::operator ==(const example::NeighborIterator &) noexcept const' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(187): note: 'bool example::NeighborIterator::operator ==(const example::NeighborIterator &) noexcept const': cannot convert argument 2 from 'const example::NeighborIteratorSentinel' to 'const example::NeighborIterator &'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(187): note: Reason: cannot convert from 'const example::NeighborIteratorSentinel' to 'const example::NeighborIterator'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(187): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(5047): note: or       'bool std::operator ==(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *const ) noexcept'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(5041): note: or       'bool std::operator ==(const std::basic_string<_Elem,_Traits,_Alloc> &,const std::basic_string<_Elem,_Traits,_Alloc> &) noexcept'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(1661): note: or       'bool std::operator ==(const std::basic_string_view<_Elem,_Traits>,const _Identity<std::basic_string_view<_Elem,_Traits>>::type) noexcept'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(1647): note: or       'bool std::operator ==(const std::basic_string_view<_Elem,_Traits>,const std::basic_string_view<_Elem,_Traits>) noexcept'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexmemory(1042): note: or       'bool std::operator ==(const std::allocator<_Ty> &,const std::allocator<_Other> &) noexcept'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includetuple(872): note: or       'bool std::operator ==(const std::tuple<_Types...> &,const std::tuple<_Types...> &)'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeiterator(488): note: or       'bool std::operator ==(const std::istreambuf_iterator<_Elem,_Traits> &,const std::istreambuf_iterator<_Elem,_Traits> &)'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeiterator(306): note: or       'bool std::operator ==(const std::istream_iterator<_Ty,_Elem,_Traits,_Diff> &,const std::istream_iterator<_Ty,_Elem,_Traits,_Diff> &) noexcept'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(4228): note: or       'bool std::operator ==(const std::move_iterator<_Iter> &,const std::move_iterator<_Iter2> &) noexcept(<expr>)'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(1686): note: or       'bool std::operator ==(const std::reverse_iterator<_BidIt> &,const std::reverse_iterator<_BidIt2> &) noexcept(<expr>)'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeutility(490): note: or       'bool std::operator ==(const std::pair<_Ty1,_Ty2> &,const std::pair<_Uty1,_Uty2> &)'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeutility(490): note: or 'bool std::operator ==(const std::pair<_Ty1,_Ty2> &,const std::pair<_Uty1,_Uty2> &)' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(1686): note: or 'bool std::operator ==(const std::reverse_iterator<_BidIt> &,const std::reverse_iterator<_BidIt2> &) noexcept(<expr>)' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(4228): note: or 'bool std::operator ==(const std::move_iterator<_Iter> &,const std::move_iterator<_Iter2> &) noexcept(<expr>)' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeiterator(306): note: or 'bool std::operator ==(const std::istream_iterator<_Ty,_Elem,_Traits,_Diff> &,const std::istream_iterator<_Ty,_Elem,_Traits,_Diff> &) noexcept' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeiterator(488): note: or 'bool std::operator ==(const std::istreambuf_iterator<_Elem,_Traits> &,const std::istreambuf_iterator<_Elem,_Traits> &)' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includetuple(872): note: or 'bool std::operator ==(const std::tuple<_Types...> &,const std::tuple<_Types...> &)' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexmemory(1042): note: or 'bool std::operator ==(const std::allocator<_Ty> &,const std::allocator<_Other> &) noexcept' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(1647): note: or 'bool std::operator ==(const std::basic_string_view<_Elem,_Traits>,const std::basic_string_view<_Elem,_Traits>) noexcept' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(1661): note: or 'bool std::operator ==(const std::basic_string_view<_Elem,_Traits>,const _Identity<std::basic_string_view<_Elem,_Traits>>::type) noexcept' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(5041): note: or 'bool std::operator ==(const std::basic_string<_Elem,_Traits,_Alloc> &,const std::basic_string<_Elem,_Traits,_Alloc> &) noexcept' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexstring(5047): note: or 'bool std::operator ==(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *const ) noexcept' [synthesized expression 'y == x']
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includeconcepts(187): note: while trying to match the argument list '(const example::NeighborIteratorSentinel, const example::NeighborIterator)'
NeighborIterator.cpp(101): error C2641: cannot deduce template arguments for 'std::ranges::subrange'
NeighborIterator.cpp(101): error C2783: 'std::ranges::subrange<_It,_Se,_Ki> std::ranges::subrange(std::true_type,_Rng &&)': could not deduce template argument for '_It'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(3745): note: see declaration of 'std::ranges::subrange'
NeighborIterator.cpp(101): error C2780: 'std::ranges::subrange<_It,_Se,_Ki> std::ranges::subrange(std::ranges::subrange<_It,_Se,_Ki>)': expects 1 arguments - 2 provided
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includexutility(3735): note: see declaration of 'std::ranges::subrange'

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Can’t create std::ranges::subrange with my iterator