std::experimental::optional<T>::optional - cppreference.com

constexpr optional() noexcept;
constexpr optional( std::experimental::nullopt_t ) noexcept;

optional( const optional& other );

optional( optional&& other ) noexcept(/* see below */);

constexpr optional( const T& value );

constexpr optional( T&& value );

template< class... Args > 
constexpr explicit optional( std::experimental::in_place_t, Args&&... args );

template< class U, class... Args >
constexpr explicit optional( std::experimental::in_place_t,
                             std::initializer_list<U> ilist, 
                             Args&&... args );

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

Exceptions

2) Throws any exception thrown by the constructor of T.

noexcept(std::is_nothrow_move_constructible<T>::value)

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

Example

#include <experimental/optional>
#include <iostream>
#include <string>

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

    std::experimental::optional<std::string> o4(std::experimental::in_place,
                                                {'a', 'b', 'c'});

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

See also

Categories:

• Pages using deprecated source tags
• Pages using deprecated enclose attributes
• Noindexed pages
• conditionally noexcept