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std::optional::optional

constexpr optional(); constexpr optional( std::nullopt_t );	(1)	(since C++17)
optional( const optional& other );	(2)	(since C++17)
optional( optional&& other );	(3)	(since C++17)
constexpr optional( const T& value );	(4)	(since C++17)
constexpr optional( T&& value );	(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)

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.

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.

4) 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 value. This constructor is constexpr if the constructor of T selected by direct-initialization is constexpr.

5) 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(value). This constructor is constexpr if the constructor of T selected by direct-initialization is constexpr.

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)...

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).... The function does not participate in the overload resolution if std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value != true

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
Type requirements
- `T` must meet the requirements of `CopyConstructible` in order to use overloads (2,4).
- `T` must meet the requirements of `MoveConstructible` in order to use overloads (3,5).

Exceptions

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)

4-7) Throws any exception thrown by the constructor of T.

Example

#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

std::optional::optional

Parameters

Exceptions

Example

See also