std::monostate (3) - Linux Manuals

std::monostate: std::monostate

NAME

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