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# std::optional::optional

From cppreference.com
< cpp‎ | utility‎ | optional

constexpr optional(); constexpr optional( std::nullopt_t ); | (1) | (since C++17)
constexpr optional( const optional& other ); | (2) | (since C++17)
constexpr optional( optional&& other ); | (3) | (since C++17)
template < class U > /* EXPLICIT */ optional( const optional<U>& other ); | (4) | (since C++17)
template < class U > /* EXPLICIT */ optional( optional<U>&& other ); | (5) | (since C++17)
template< class... Args > constexpr explicit optional( std::in_place_t, Args&&... args ); | (6) | (since C++17)
template< class U, class... Args > constexpr explicit optional( std::in_place_t, std::initializer_list<U> ilist, Args&&... args ); | (7) | (since C++17)
template < class U = value_type > /* EXPLICIT */ constexpr optional( U&& value ); | (8) | (since C++17)

Constructs a new `optional` object.

1) Constructs the object that does not contain a value.
2) Copy constructor: If `other` contains a value, initializes the contained value as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression *other. If `other` does not contain a value, constructs an object that does not contain a value. This constructor is defined as deleted if std::is_copy_constructible_v<T> is false. It is a constexpr constructor if std::is_trivially_copy_constructible_v<T> is true.
3) Move constructor: If `other` contains a value, initializes the contained value as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression std::move(*other) and does not make `other` empty: a moved-from optional still contains a value, but the value itself is moved from. If `other` does not contain a value, constructs an object that does not contain a value. This constructor does not participate in overload resolution unless std::is_move_constructible_v<T> is true. It is a constexpr constructor if std::is_trivially_move_constructible_v<T> is true.
4) Converting copy constructor: If `other` doesn't contain a value, constructs an optional object that does not contain a value. Otherwise, constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression *other. This constructor does not participate in overload resolution unless the following conditions are met:

- std::is_constructible_v<T, const U&> is true.
- `T` is not constructible or convertible from any expression of type (possibly const) std::optional<U>, i.e., the following 8 type traits are all false:
  - std::is_constructible_v<T, std::optional<U>&>
  - std::is_constructible_v<T, const std::optional<U>&>
  - std::is_constructible_v<T, std::optional<U>&&>
  - std::is_constructible_v<T, const std::optional<U>&&>
  - std::is_convertible_v<std::optional<U>&, T>
  - std::is_convertible_v<const std::optional<U>&, T>
  - std::is_convertible_v<std::optional<U>&&, T>
  - std::is_convertible_v<const std::optional<U>&&, T>

This constructor is explicit if and only if std::is_convertible_v<const U&, T> is false.
5) Converting move constructor: If `other` doesn't contain a value, constructs an optional object that does not contain a value. Otherwise, constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression std::move(*other). This constructor does not participate in overload resolution unless the following conditions are met:

- std::is_constructible_v<T, U&&> is true.
- `T` is not constructible or convertible from any expression of type (possibly const) std::optional<U>, i.e., the following 8 type traits are all false:
  - std::is_constructible_v<T, std::optional<U>&>
  - std::is_constructible_v<T, const std::optional<U>&>
  - std::is_constructible_v<T, std::optional<U>&&>
  - std::is_constructible_v<T, const std::optional<U>&&>
  - std::is_convertible_v<std::optional<U>&, T>
  - std::is_convertible_v<const std::optional<U>&, T>
  - std::is_convertible_v<std::optional<U>&&, T>
  - std::is_convertible_v<const std::optional<U>&&, T>

This constructor is explicit if and only if std::is_convertible_v<U&&, T> is false.
6) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` from the arguments std::forward<Args>(args).... If the selected constructor of `T` is a constexpr constructor, this constructor is a constexpr constructor. The function does not participate in the overload resolution unless std::is_constructible_v<T, Args...> is true
7) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` from the arguments ilist, std::forward<Args>(args).... If the selected constructor of `T` is a constexpr constructor, this constructor is a constexpr constructor. The function does not participate in the overload resolution unless std::is_constructible_v<T, std::initializer_list<U>&, Args&&...> is true
8) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` (where `T` = `value_type`) with the expression std::forward<U>(value). If the selected constructor of `T` is a constexpr constructor, this constructor is a constexpr constructor. This constructor does not participate in overload resolution unless std::is_constructible_v<T, U&&> is true and std::decay_t<U> is neither std::in_place_t nor std::optional<T>. This constructor is explicit if and only if std::is_convertible_v<U&&, T> is false.

### [edit] Parameters

other | - | another `optional` object whose contained value to copy
value | - | value to initialize the contained value with
args... | - | arguments to initialize the contained value with
ilist | - | initializer list to initialize the contained value with

### [edit] Exceptions

1)
`noexcept` specification:
noexcept
2) Throws any exception thrown by the constructor of `T`.
3) Throws any exception thrown by the constructor of `T`. Has the following `noexcept` declaration:
`noexcept` specification:
noexcept(std::is_nothrow_move_constructible<T>::value)
.
4-8) Throws any exception thrown by the constructor of `T`.

### [edit] Example

Run this code

    #include <optional>
    #include <iostream>
    #include <string>
    int main()
    {
        std::optional<int> o1, // empty
                           o2 = 1, // init from rvalue
                           o3 = o2; // copy-constructor

        // calls string( initializer_list<CharT> ) constructor
        std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'});

        // calls string( size_type count, CharT ch ) constructor
        std::optional<std::string> o5(std::in_place, 3, 'A');

        std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << ' ' << *o5  << '\n';
    }

Output:

    1 1 abc AAA

### [edit] See also

make_optional (C++17) | creates an `optional` object (function template) [edit]

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