Assignable wrapper (C++20) - cppreference.com (original) (raw)

ranges::single_view, ranges::repeat_view,(since C++23) and range adaptors that store an invocable object are specified in terms of an exposition-only class template _copyable-box_(until C++23)_movable-box_(since C++23). The name shown here is for exposition purposes only.

The wrapper behaves exactly like std::optional<T>, except that the default constructor, copy assignment operator, and move assignment operator are (conditionally) different from those of std::optional, which augments T with assignability when needed and makes it always satisfy copyableor movable(since C++23).

Contents

• 1 Template parameters
• 2 Member functions
• 3 Default constructor
• 4 Assignment operators
• 5 Members identical to std::optional
  • 5.1 Member functions
    * 5.1.1 Observers
    * 5.1.2 Modifiers
  • 5.2 Notes
  • 5.3 Defect reports
  • 5.4 See also

[edit] Template parameters

[edit] Member functions

Default constructor

The default constructor is provided if and only if T models default_initializable.

A default-constructed wrapper contains a value-initialized T object.

Assignment operators

| (1) | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | --------------------------- | | constexpr /*copyable-box*/& operator=(const /*copyable-box*/& other); noexcept(/* see below */); | | (since C++20) (until C++23) | | constexpr /*movable-box*/& operator=(const /*movable-box*/& other); noexcept(/* see below */) requires std::copy_constructible<T>; | | (since C++23) | | (2) | | | | constexpr /*copyable-box*/& operator=(/*copyable-box*/&& other) noexcept(std::is_nothrow_move_constructible_v<T>); | | (since C++20) (until C++23) | | constexpr /*movable-box*/& operator=(/*movable-box*/&& other) noexcept(std::is_nothrow_move_constructible_v<T>); | | (since C++23) |

Members identical to std::optional

Member functions

[edit] Notes

A _copyable-box_(until C++23)_movable-box_(since C++23) does not contain a value only if

• T does not model movable or copyable, and an exception is thrown on move assignment or copy assignment respectively, or
• it is initialized/assigned from another valueless wrapper.

Before P2325R3, the wrapper was called _semiregular-box_ in the standard and always satisfied semiregular, as the default constructor was always provided (which might construct a valueless wrapper).

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also