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std::optional::optional

From cppreference.com
< cpp‎ | utility‎ | 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

Contents

[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
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).

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

[edit] Example

#include <optional>
#include <iostream>
#include <string>
int main()
{
    std::optional<int> o1, // empty
                       o2 = 1, // init from rvalue
                       o3 = o2; // copy-constructor
 
    std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'});
 
    std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << '\n';
}

Output:

1 1 abc

[edit] See also

creates an optional object
(function template) [edit]