std::optional::optional

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std::optional
 
constexpr optional() noexcept;
constexpr optional( std::nullopt_t ) noexcept;
 (1) (since C++17)
constexpr optional( const optional& other );
 (2) (since C++17)
constexpr optional( optional&& other ) noexcept(/* see below */);
 (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

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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