# std::monostate

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< cpp‎ | utility‎ | variant

Defined in header `<variant>` |  |
struct monostate { }; |  | (since C++17)

Unit type intended for use as a well-behaved empty alternative in std::variant. In particular, a variant of non-default-constructible types may list `std::monostate` as its first alternative: this makes the variant itself default-constructible.

### Member functions

(constructor) (implicitly declared) | trivial implicit default/copy/move constructor (public member function)
(destructor) (implicitly declared) | trivial implicit destructor (public member function)
operator= (implicitly declared) | trivial implicit copy/move assignment (public member function)

### Non-member functions

##

constexpr bool operator<(monostate, monostate) noexcept { return false; }

constexpr bool operator>(monostate, monostate) noexcept { return false; }
constexpr bool operator<=(monostate, monostate) noexcept { return true; }
constexpr bool operator>=(monostate, monostate) noexcept { return true; }
constexpr bool operator==(monostate, monostate) noexcept { return true; }

constexpr bool operator!=(monostate, monostate) noexcept { return false; }

All instances of `std::monostate` compare equal.

### Helper classes

## std::hash<std::monostate>

template <> struct std::hash<monostate>; |  |

Specializes the std::hash algorithm for `std::monostate`.

### Example

Run this code

    #include <variant>
    #include <iostream>

    struct S
    {
        S(int i) : i(i) {}
        int i;
    };

    int main() {

        // Without the monostate type this declaration will fail.
        // This is because S is not default-constructible.

        std::variant<std::monostate, S> var;

        // var.index() is now 0 - the first element
        // std::get<S> will throw! We need to assign a value

        var = 12;

        std::cout << std::get<S>(var).i << '\n';
    }

Output:

    12

### See also

(constructor) | constructs the variant object (public member function)

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