std::experimental::optional<T>:: value

From cppreference.com
C++
Experimental
Library fundamentals
std::experimental::optional
constexpr T & value ( ) & ;
constexpr const T & value ( ) const & ;
(1) (library fundamentals TS)
constexpr T && value ( ) && ;
constexpr const T && value ( ) const && ;
(2) (library fundamentals TS)

Returns the contained value.

1) Equivalent to return bool ( * this ) ? * val : throw bad_optional_access ( ) ; .
2) Equivalent to return bool ( * this ) ? std :: move ( * val ) : throw bad_optional_access ( ) ; .

Parameters

(none)

Return value

A reference to the contained value.

Exceptions

std::experimental::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 <experimental/optional>
#include <iostream>
 
int main()
{
    std::experimental::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

See also

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