22 General utilities library [utilities]

22.4 Tuples [tuple]

22.4.9 Relational operators [tuple.rel]

template<class... TTypes, class... UTypes>
  constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
template<class... TTypes, tuple-like UTuple>
  constexpr bool operator==(const tuple<TTypes...>& t, const UTuple& u);

For the first overload let UTuple be tuple<UTypes...>.

Constraints: For all i, where 0 ≤ i < sizeof...(TTypes), get(t) == get(u) is a valid expression and decltype(get(t) == get(u)) models boolean-testable.

sizeof...(TTypes) equals tuple_size_v<UTuple>.

Returns: true if get(t) == get(u) for all i, otherwise false.

[Note 1:

If sizeof...(TTypes) equals zero, returns true.

— end note]

Remarks:

(4.1)
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.
(4.2)
The second overload is to be found via argument-dependent lookup ([basic.lookup.argdep]) only.

🔗

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);
template<class... TTypes, tuple-like UTuple>
  constexpr common_comparison_category_t<synth-three-way-result<TTypes, Elems>...>
    operator<=>(const tuple<TTypes...>& t, const UTuple& u);

For the second overload, Elems denotes the pack of types tuple_element_t<0, UTuple>, tuple_element_t<1, UTuple>, …, tuple_element_t<tuple_size_v<UTuple> - 1, UTuple>.

Effects: Performs a lexicographical comparison between t and u.

If sizeof...(TTypes) equals zero, returns strong_ordering::equal.

Otherwise, equivalent to: if (auto c = synth-three-way(get<0>(t), get<0>(u)); c != 0) return c; return

t_{t a i l}

<=>

u_{t a i l}

; where

r_{t a i l}

for some r is a tuple containing all but the first element of r.

Remarks: The second overload is to be found via argument-dependent lookup ([basic.lookup.argdep]) only.

[Note 2:

The above definition does not require t

_{t a i l}

(or u

_{t a i l}

) to be constructed.

It might not even be possible, as t and u are not required to be copy constructible.

Also, all comparison operator functions are short circuited; they do not perform element accesses beyond what is needed to determine the result of the comparison.

— end note]