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

Possible output:

optional<T>::value: not engaged