std::monostate (3) Linux Manual Page

std::monostate – std::monostate

Synopsis

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 trivial implicit default/copy/move constructor
                      (public member function)
(implicitly declared)

destructor trivial implicit destructor
                      (public member function)
(implicitly declared)

operator= trivial implicit copy/move assignment
                      (public member function)
(implicitly declared)

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

              constructs the variant object
constructor (public member function)