5 Quantities and units library [quantities]

5.4 Reference [qty.ref]

In this subclause and [qty.is.child.of], let the kind of quantity (IEC 60050, 112-01-04) hierarchy of q be the tuple

h (q) = (q, q.par, q.par.par, \dots)

, where par is parent.

template<QuantitySpec auto A, QuantitySpec auto B>
consteval auto get-common-base();  // exposition only

Let

(2.1)
$a_{s}$ be the number of elements in h(A),
(2.2)
$b_{s}$ be the number of elements in h(B),
(2.3)
s be $min (a_{s}, b_{s})$ ,
(2.4)
A be a tuple of the last s elements of h(A), and
(2.5)
B be a tuple of the last s elements of h(B).

Effects: Looks for x, the first pair-wise equal element in A and B.

Returns: std::optional(x), if x is found, and std::optional<unspecified>() otherwise.