24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.2 Semiregular wrapper [range.semi.wrap]

1
#
Many types in this subclause are specified in terms of an exposition-only class template semiregular-box.
semiregular-box<T> behaves exactly like optional<T> with the following differences:
  • (1.1)
    semiregular-box<T> constrains its type parameter T with copy_­constructible<T> && is_­object_­v<T>.
  • (1.2)
    If T models default_­initializable, the default constructor of semiregular-box<T> is equivalent to: 
    constexpr semiregular-box() noexcept(is_nothrow_default_constructible_v<T>)
  : semiregular-box{in_place}
{ }
  • (1.3)
    If assignable_­from<T&, const T&> is not modeled, the copy assignment operator is equivalent to: 
    semiregular-box& operator=(const semiregular-box& that)
  noexcept(is_nothrow_copy_constructible_v<T>)
{
  if (that) emplace(*that);
  else reset();
  return *this;
}
  • (1.4)
    If assignable_­from<T&, T> is not modeled, the move assignment operator is equivalent to: 
    semiregular-box& operator=(semiregular-box&& that)
  noexcept(is_nothrow_move_constructible_v<T>)
{
  if (that) emplace(std::move(*that));
  else reset();
  return *this;
}