17 Language support library [support] (original) (raw)

17.11 Comparisons [cmp]

17.11.6 Comparison algorithms [cmp.alg]

Given subexpressions E and F, the expression strong_­order(E, F)is expression-equivalent ([defns.expression-equivalent]) to the following:

• If the decayed types of E and F differ,strong_­order(E, F) is ill-formed.
• Otherwise, strong_­ordering(strong_­order(E, F)) if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of std​::​strong_­order.
• Otherwise, if the decayed type T of E is a floating-point type, yields a value of type strong_­ordering that is consistent with the ordering observed by T's comparison operators, and if numeric_­limits<T>​::​is_­iec559 is true, is additionally consistent with the totalOrder operation as specified in ISO/IEC/IEEE 60559.
• Otherwise, strong_­ordering(compare_­three_­way()(E, F)) if it is a well-formed expression.
• Otherwise, strong_­order(E, F) is ill-formed.
  [ Note
  : This case can result in substitution failure when strong_­order(E, F) appears in the immediate context of a template instantiation. — end note
  ]

Given subexpressions E and F, the expression weak_­order(E, F)is expression-equivalent ([defns.expression-equivalent]) to the following:

• If the decayed types of E and F differ,weak_­order(E, F) is ill-formed.
• Otherwise, weak_­ordering(weak_­order(E, F)) if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of std​::​weak_­order.
• Otherwise, if the decayed type T of E is a floating-point type, yields a value of type weak_­ordering that is consistent with the ordering observed by T's comparison operators and strong_­order, and if numeric_­limits<T>​::​is_­iec559 is true, is additionally consistent with the following equivalence classes, ordered from lesser to greater:
  • together, all negative NaN values;
  • negative infinity;
  • each normal negative value;
  • each subnormal negative value;
  • together, both zero values;
  • each subnormal positive value;
  • each normal positive value;
  • positive infinity;
  • together, all positive NaN values.
• Otherwise, weak_­ordering(compare_­three_­way()(E, F)) if it is a well-formed expression.
• Otherwise, weak_­ordering(strong_­order(E, F)) if it is a well-formed expression.
• Otherwise, weak_­order(E, F) is ill-formed.
  [ Note
  : This case can result in substitution failure when std​::​weak_­order(E, F) appears in the immediate context of a template instantiation. — end note
  ]

Given subexpressions E and F, the expression partial_­order(E, F)is expression-equivalent ([defns.expression-equivalent]) to the following:

• If the decayed types of E and F differ,partial_­order(E, F) is ill-formed.
• Otherwise, partial_­ordering(partial_­order(E, F)) if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of std​::​partial_­order.
• Otherwise, partial_­ordering(compare_­three_­way()(E, F)) if it is a well-formed expression.
• Otherwise, partial_­ordering(weak_­order(E, F)) if it is a well-formed expression.
• Otherwise, partial_­order(E, F) is ill-formed.
  [ Note
  : This case can result in substitution failure when std​::​partial_­order(E, F) appears in the immediate context of a template instantiation. — end note
  ]

Given subexpressions E and F, the expression compare_­strong_­order_­fallback(E, F)is expression-equivalent ([defns.expression-equivalent]) to:

• If the decayed types of E and F differ,compare_­strong_­order_­fallback(E, F) is ill-formed.
• Otherwise, strong_­order(E, F) if it is a well-formed expression.
• Otherwise, if the expressions E == F and E < F are both well-formed and convertible to bool,
  E == F ? strong_ordering::equal :
  E < F ? strong_ordering::less :
  strong_ordering::greater

except that E and F are evaluated only once.

• Otherwise, compare_­strong_­order_­fallback(E, F) is ill-formed.

Given subexpressions E and F, the expression compare_­weak_­order_­fallback(E, F)is expression-equivalent ([defns.expression-equivalent]) to:

• If the decayed types of E and F differ,compare_­weak_­order_­fallback(E, F) is ill-formed.
• Otherwise, weak_­order(E, F) if it is a well-formed expression.
• Otherwise, if the expressions E == F and E < F are both well-formed and convertible to bool,
  E == F ? weak_ordering::equivalent :
  E < F ? weak_ordering::less :
  weak_ordering::greater

except that E and F are evaluated only once.

• Otherwise, compare_­weak_­order_­fallback(E, F) is ill-formed.

Given subexpressions E and F, the expression compare_­partial_­order_­fallback(E, F)is expression-equivalent ([defns.expression-equivalent]) to:

• If the decayed types of E and F differ,compare_­partial_­order_­fallback(E, F) is ill-formed.
• Otherwise, partial_­order(E, F) if it is a well-formed expression.
• Otherwise, if the expressions E == F and E < F are both well-formed and convertible to bool,
  E == F ? partial_ordering::equivalent :
  E < F ? partial_ordering::less :
  F < E ? partial_ordering::greater :
  partial_ordering::unordered

except that E and F are evaluated only once.

• Otherwise, compare_­partial_­order_­fallback(E, F) is ill-formed.