26 Ranges library [ranges]

Many types in this subclause are specified in terms of an exposition-only class template movable-box.

movable-box<T> behaves exactly like optional<T> with the following differences:

• (1.1)
  movable-box<T> constrains its type parameter T with move_constructible<T> && is_object_v<T>.
• (1.2)
  The default constructor of movable-box<T> is equivalent to: constexpr movable-box() noexcept(is_nothrow_default_constructible_v<T>) requires default_initializable<T> : movable-box{in_place} {}
• (1.3)
  If copyable<T> is not modeled, the copy assignment operator is equivalent to: constexpr movable-box& operator=(const movable-box& that) noexcept(is_nothrow_copy_constructible_v<T>) requires copy_constructible<T> { if (this != addressof(that)) { if (that) emplace(*that); else reset(); } return *this; }
• (1.4)
  If movable<T> is not modeled, the move assignment operator is equivalent to: constexpr movable-box& operator=(movable-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(std::move(*that)); else reset(); } return *this; }

Recommended practice:

• (2.1)
  If copy_constructible<T> is true, movable-box<T> should store only a T if either T models copyable, or is_nothrow_move_constructible_v<T> && is_nothrow_copy_constructible_v<T> is true.
• (2.2)
  Otherwise, movable-box<T> should store only a T if either T models movable or is_nothrow_move_constructible_v<T> is true.