Issue 928 - WG21 LWG Issues
Title
Wrong concepts used for tuple's comparison operators
Status
nad concepts
Section
[tuple.rel]
Submitter
Joe Gottman

Created on 2008-10-28.00:00:00 last changed 172 months ago

Messages

msg4409 (view)
Date: 2010-10-21.18:28:33

Proposed resolution:

In [tuple.general] and [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...>&);
msg4408 (view)
Date: 2010-10-21.18:28:33

[ Post Summit: ]

Recommend Tentatively Ready.

msg4407 (view)
Date: 2008-10-28.00:00:00

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.

History
Date User Action Args
2010-10-21 18:28:33adminsetmessages: + msg4409
2010-10-21 18:28:33adminsetmessages: + msg4408
2008-10-28 00:00:00admincreate