std::optional<T>::value

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< cpp‎ | utility‎ | optional
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std::optional
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

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 <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

 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)
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