The reference type.
using const_reference = param;
Objects of this type represent a single key and value pair in a query string
where a key is always present and may be empty, while the presence of a
value is indicated by
has_value equal to true. An empty
value is distinct from no value. Depending on where the object was obtained,
the strings may or may not contain percent escapes. For most usages, key
comparisons are case-sensitive and duplicate keys in a query are possible.
However, it is the authority that has final control over how the query
query-params = query-param *( "&" query-param ) query-param = key [ "=" value ] key = *qpchar value = *( qpchar / "=" )
This is the type of value returned when iterators of the view are dereferenced.