std::experimental::optional<T>::optional

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms 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)

Extensions for parallelism 2 (parallelism TS v2)

Extensions for concurrency (concurrency TS)

Concepts (concepts TS)

Ranges (ranges 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() noexcept; constexpr optional( std::experimental::nullopt_t ) noexcept;	(1)	(library fundamentals TS)
optional( const optional& other );	(2)	(library fundamentals TS)
optional( optional&& other ) noexcept(/* see below */);	(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

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)