W3cubDocs

/C++

std::optional<T>::value

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

#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

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

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/utility/optional/value