std::optional<T>::value

 

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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

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

constexpr T& value()&; constexprconst T& value()const&;	(1)	(since C++17)
constexpr T&& value()&&; constexprconst T&& value()const&&;	(2)	(since C++17)

If *this contains a value, returns a reference to the contained value.

Otherwise, throws a std::bad_optional_access exception.

Parameters

(none)

Return value

A reference to the contained value.

Exceptions

std::bad_optional_access if *this does not contain a value.

Notes

The dereference operator operator*() does not check if this optional contains a value, which may be more efficient than value().

Example

Run this code

#include <iostream>#include <optional>   int main(){std::optional<int> opt ={};   try{[[maybe_unused]]int n = opt.value();}catch(conststd::bad_optional_access& e){std::cout<< e.what()<<'\n';}   try{ opt.value()=42;}catch(conststd::bad_optional_access& e){std::cout<< e.what()<<'\n';}   opt =43;std::cout<<*opt <<'\n';   opt.value()=44;std::cout<< opt.value()<<'\n';}

Output:

bad optional access bad optional access 43 44

See also

value_or	returns the contained value if available, another value otherwise (public member function)
operator->operator*	accesses the contained value (public member function)
bad_optional_access (C++17)	exception indicating checked access to an optional that doesn't contain a value (class)