std::optional::value (3) Linux Manual Page

std::optional<T>::value – std::optional<T>::value

Synopsis

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

    constexpr const T &value() const &;

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

    constexpr const T &&value() const &&;

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 {
        int n = opt.value();
    } catch (const std::bad_optional_access &e) {
        std::cout << e.what() << '\n';
    }
}

Possible output:

  bad optional access

See also

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