5 Quantities and units library [quantities]

5.6 Quantity [qty]

5.6.13 Value comparison [qty.val.cmp]

friend constexpr bool is_eq_zero(const quantity& q) requires see below;
friend constexpr bool is_neq_zero(const quantity& q) requires see below;
friend constexpr bool is_lt_zero(const quantity& q) requires see below;
friend constexpr bool is_gt_zero(const quantity& q) requires see below;
friend constexpr bool is_lteq_zero(const quantity& q) requires see below;
friend constexpr bool is_gteq_zero(const quantity& q) requires see below;

Let is_F_zero be the function name.

Returns:

(2.1)
If F is eq, returns q == zero().
(2.2)
Otherwise, if F is neq, returns q != zero().
(2.3)
Otherwise, returns is_F(q <=> zero()) (N4971, [compare.syn]).

Remarks: Let C be std::equality_comparable_with if F is eq or neq, and std::three_way_comparable_with otherwise.

The expression in the requires-clause is equivalent to: requires { { T::zero() } -> C<quantity>; }