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std::optional<T>::value - cppreference.com

From cppreference.com

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 <iostream>
#include <optional>

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