tmp/tmp8kml2_85/{from.md → to.md}
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| 1 |
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### Three-way comparison <a id="class.spaceship">[[class.spaceship]]</a>
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The *synthesized three-way comparison* of type `R` [[cmp.categories]] of
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glvalues `a` and `b` of the same type is defined as follows:
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- If `a <=> b` is usable [[class.compare.default]],
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`static_cast<R>(a <=> b)`.
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- Otherwise, if overload resolution for `a <=> b` is performed and finds
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at least one viable candidate, the synthesized three-way comparison is
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not defined.
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- Otherwise, if `R` is not a comparison category type, or either the
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expression `a == b` or the expression `a < b` is not usable, the
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synthesized three-way comparison is not defined.
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- Otherwise, if `R` is `strong_ordering`, then
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``` cpp
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a == b ? strong_ordering::equal :
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a < b ? strong_ordering::less :
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strong_ordering::greater
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```
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- Otherwise, if `R` is `weak_ordering`, then
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``` cpp
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a == b ? weak_ordering::equivalent :
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a < b ? weak_ordering::less :
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weak_ordering::greater
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```
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- Otherwise (when `R` is `partial_ordering`),
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``` cpp
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a == b ? partial_ordering::equivalent :
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a < b ? partial_ordering::less :
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b < a ? partial_ordering::greater :
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partial_ordering::unordered
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```
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[*Note 1*: A synthesized three-way comparison may be ill-formed if
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overload resolution finds usable candidates that do not otherwise meet
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the requirements implied by the defined expression. — *end note*]
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Let `R` be the declared return type of a defaulted three-way comparison
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operator function, and let `xᵢ` be the elements of the expanded list of
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subobjects for an object `x` of type `C`.
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- If `R` is `auto`, then let cvᵢ~`Rᵢ` be the type of the expression
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`xᵢ`` <=> ``xᵢ`. The operator function is defined as deleted if that
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expression is not usable or if `Rᵢ` is not a comparison category type
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[[cmp.categories.pre]] for any i. The return type is deduced as the
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common comparison type (see below) of `R₀`, `R₁`, …, `R_n-1`.
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- Otherwise, `R` shall not contain a placeholder type. If the
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synthesized three-way comparison of type `R` between any objects `xᵢ`
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and `xᵢ` is not defined, the operator function is defined as deleted.
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The return value `V` of type `R` of the defaulted three-way comparison
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operator function with parameters `x` and `y` of the same type is
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determined by comparing corresponding elements `xᵢ` and `yᵢ` in the
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expanded lists of subobjects for `x` and `y` (in increasing index order)
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until the first index i where the synthesized three-way comparison of
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type `R` between `xᵢ` and `yᵢ` yields a result value `vᵢ` where
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`vᵢ` `!=` 0, contextually converted to `bool`, yields `true`; `V` is a
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copy of `vᵢ`. If no such index exists, `V` is
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`static_cast<R>(std::strong_ordering::equal)`.
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The *common comparison type* `U` of a possibly-empty list of n
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comparison category types `T₀`, `T₁`, …, `T_n-1` is defined as follows:
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- If at least one `Tᵢ` is `std::partial_ordering`, `U` is
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`std::partial_ordering` [[cmp.partialord]].
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- Otherwise, if at least one `Tᵢ` is `std::weak_ordering`, `U` is
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`std::weak_ordering` [[cmp.weakord]].
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- Otherwise, `U` is `std::strong_ordering` [[cmp.strongord]].
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\[*Note 2*: In particular, this is the result when n is
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0. — *end note*]
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