std::optional<T>::value

 

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std::optional

Member functions

optional::optional (C++17)

optional::~optional (C++17)

optional::operator= (C++17)

Observers

optional::operator->optional::operator* (C++17)(C++17)

optional::operator booloptional::has_value (C++17)(C++17)

optional::value (C++17)

optional::value_or (C++17)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace (C++17)

optional::swap (C++17)

optional::reset (C++17)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)

make_optional (C++17)

swap (C++17)

Deduction guides

Helper classes

hash (C++17)

nullopt_t (C++17)

in_place_t (C++17)

bad_optional_access (C++17)

Helper objects

nullopt (C++17)

in_place (C++17)

 

constexpr T& value() &; constexpr const T& value() const &;	(1)	(since C++17)
constexpr T&& value() &&; constexpr const 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 <optional>
#include <iostream>
int main()
{
    std::optional<int> opt = {};
 
    try {
        [[maybe_unused]] int n = opt.value();
    } catch(const std::bad_optional_access& e) {
        std::cout << e.what() << '\n';
    }
    try {
        opt.value() = 42;
    } catch(const std::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)