std::optional::optional

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std::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:
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:
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.

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

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.

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

See also

(C++17)
 creates an optional object
(function template)
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