std::optional

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

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Algorithms library

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Numerics library

Input/output library

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Regular expressions library (C++11)

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Thread support library (C++11)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Elementary string conversions

to_chars (C++17)

from_chars (C++17)

chars_format (C++17)

Type operations

declval (C++11)

as_const (C++17)

 

std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

optional::reset

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

 

Defined in header <optional>
template< class T > class optional;		(since C++17)

The class template std::optional manages an optional contained value, i.e. a value that may or may not be present.

A common use case for optional is the return value of a function that may fail. As opposed to other approaches, such as std::pair<T,bool>, optional handles expensive-to-construct objects well and is more readable, as the intent is expressed explicitly.

Any instance of optional<T> at any given point in time either contains a value or does not contain a value.

If an optional<T> contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. no dynamic memory allocation ever takes place. Thus, an optional object models an object, not a pointer, even though the operator*() and operator->() are defined.

When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value and false if it does not contain a value.

The optional object contains a value in the following conditions:

• The object is initialized with a value of type T
• The object is assigned from another optional that contains a value.

The object does not contain a value in the following conditions:

• The object is default-initialized.
• The object is initialized with a value of std::nullopt_t or an optional object that does not contain a value.
• The object is assigned from a value of std::nullopt_t or from an optional that does not contain a value

There are no optional references, a program is ill-formed if it instantiates optional with a reference type.

Template parameters

T

-

the type of the value to manage initialization state for. The type must meet the requirements of Destructible

Member types

Member type	Definition
value_type	T

Member functions

(constructor)	constructs the optional object (public member function)
(destructor)	destroys the contained value, if there is one (public member function)
operator=	assigns contents (public member function)
Observers
operator->operator*	accesses the contained value (public member function)
operator boolhas_value	checks whether the object contains a value (public member function)
value	returns the contained value (public member function)
value_or	returns the contained value if available, another value otherwise (public member function)
Modifiers
swap	exchanges the contents (public member function)
reset	destroys any contained value (public member function)
emplace	constructs the contained value in-place (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>= (C++17)	compares optional objects (function template)
make_optional (C++17)	creates an optional object (function template)
std::swap(std::optional) (C++17)	specializes the std::swap algorithm (function)

Helper classes

std::hash<std::optional> (C++17)	specializes the std::hash algorithm (class template specialization)
nullopt_t (C++17)	indicator of optional type with uninitialized state (class)
bad_optional_access (C++17)	exception indicating checked access to an optional that doesn't contain a value (class)

Helpers

nullopt (C++17)	an object of type nullopt_t (constant)
in_place in_place_type in_place_index in_place_t in_place_type_t in_place_index_t (C++17)	in-place construction tag (class template)

Example

Run this code

#include <string>
#include <iostream>
#include <optional>
 
// optional can be used as the return type of a factory that may fail
std::optional<std::string> create(bool b) {
    if(b)
        return "Godzilla";
    else
        return {};
}
 
int main()
{
    std::cout << "create(false) returned "
              << create(false).value_or("empty") << '\n';
 
    // optional-returning factory functions are usable as conditions of while and if
    if(auto str = create(true)) {
        std::cout << "create(true) returned " << *str << '\n';
    }
}

Output:

create(false) returned empty
create(true) returned Godzilla