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Showing content from https://cplusplus.github.io/LWG/issue928 below:

Wrong concepts used for tuple's comparison operators

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of NAD Concepts status.

Section: 22.4.9 [tuple.rel] Status: NAD Concepts Submitter: Joe Gottman Opened: 2008-10-28 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [tuple.rel].

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Discussion:

In the latest working draft for C++0x, tuple's operator== and operator< are declared as

template<class... TTypes, class... UTypes> 
  requires EqualityComparable<TTypes, UTypes>... 
  bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

and

template<class... TTypes, class... UTypes> 
  requires LessThanComparable<TTypes, UTypes>... 
  bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

But the concepts EqualityComparable and LessThanComparable only take one parameter, not two. Also, even if LessThanComparable could take two parameters, the definition of tuple::operator<() should also require

LessThanComparable<UTypes, TTypes>... // (note the order) 

since the algorithm for tuple::operator< is the following (pseudo-code)

for (size_t N = 0; N < sizeof...(TTypes); ++N) { 
    if (get<N>(t) < get<N>(u) return true; 
    else if ((get<N>(u) < get<N>(t)) return false; 
} 

return false; 

Similar problems hold for tuples's other comparison operators.

[ Post Summit: ]

Recommend Tentatively Ready.

Proposed resolution:

In 22.4.1 [tuple.general] and 22.4.9 [tuple.rel] change:

template<class... TTypes, class... UTypes>
  requires EqualityComparableHasEqualTo<TTypes, UTypes>...
  bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>...
  bool operator<(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires EqualityComparableHasEqualTo<TTypes, UTypes>...
  bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<UTTypes, TUTypes>... && HasLess<UTypes, TTypes>...
  bool operator>(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<UTTypes, TUTypes>... && HasLess<UTypes, TTypes>...
  bool operator<=(const tuple<TTypes...>&, const tuple<UTypes...>&);

template<class... TTypes, class... UTypes>
  requires LessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>...
  bool operator>=(const tuple<TTypes...>&, const tuple<UTypes...>&);

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