20 General utilities library [utilities]

20.5 Tuples [tuple]

20.5.8 Relational operators [tuple.rel]

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template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
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Mandates: For all i, where , get<i>(t) == get<i>(u) is a valid expression returning a type that is convertible to bool.
sizeof...(TTypes) equals sizeof...(UTypes).
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Returns: true if get<i>(t) == get<i>(u) for all i, otherwise false.
For any two zero-length tuples e and f, e == f returns true.
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Effects: The elementary comparisons are performed in order from the zeroth index upwards.
No comparisons or element accesses are performed after the first equality comparison that evaluates to false.
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template<class... TTypes, class... UTypes> constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...> operator<=>(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
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Effects: Performs a lexicographical comparison between t and u.
For any two zero-length tuples t and u, t <=> u returns strong_­ordering​::​equal.
Otherwise, equivalent to: 
if (auto c = synth-three-way(get<0>(t), get<0>(u)); c != 0) return c;
return  <=> ;
where for some tuple r is a tuple containing all but the first element of r.
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[Note
:
The above definition does not require t (or u) to be constructed.
It may not even be possible, as t and u are not required to be copy constructible.
Also, all comparison functions are short circuited; they do not perform element accesses beyond what is required to determine the result of the comparison.
— end note
]