std::experimental::optional::optional

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Technical specifications

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals 2 TS)

Extensions for parallelism (parallelism TS)

Concepts (concepts TS)

Extensions for concurrency (concurrency TS)

Special mathematical functions (special math TR)

 

Library fundamentals

experimental::optional

experimental::any

experimental::basic_string_view

experimental::sample

experimental::shared_ptr

experimental::weak_ptr

experimental::apply

experimental::invocation_type experimental::raw_invocation_type

experimental::search

experimental::default_searcher experimental::make_default_searcher

experimental::boyer_moore_searcher experimental::make_boyer_moore_searcher

experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher

type-erased and polymorphic allocators

variable templates for type traits

 

std::experimental::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator bool

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

Non-member functions

operator==operator!=operator<operator<=operator>operator>=

make_optional

swap

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

constexpr optional(); constexpr optional( std::experimental::nullopt_t );	(1)	(library fundamentals TS)
optional( const optional& other );	(2)	(library fundamentals TS)
optional( optional&& other );	(3)	(library fundamentals TS)
constexpr optional( const T& value );	(4)	(library fundamentals TS)
constexpr optional( T&& value );	(5)	(library fundamentals TS)
template< class... Args > constexpr explicit optional( std::experimental::in_place_t, Args&&... args );	(6)	(library fundamentals TS)
template< class U, class... Args > constexpr explicit optional( std::experimental::in_place_t,                             std::initializer_list<U> ilist,                             Args&&... args );	(7)	(library fundamentals TS)

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

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)

.

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

Example

Run this code

#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';
}

Output:

1 1 abc

See also

make_optional	creates an optional object (function template)