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

Non-member functions

std::operator==, !=, <, <=, >, >=, <=>(std::monostate)

constexpr bool operator==(monostate, monostate) noexcept { return (since C++17)
true; }
constexpr bool operator!=(monostate, monostate) noexcept { return
false; }

constexpr bool operator<(monostate, monostate) noexcept { return
false; }
constexpr bool operator>(monostate, monostate) noexcept { return (since C++17)
false; } (until C++20)
constexpr bool operator<=(monostate, monostate) noexcept { return
true; }

constexpr bool operator>=(monostate, monostate) noexcept { return
true; }
constexpr std::strong_ordering operator<=>(monostate, monostate)
noexcept (since C++20)
{ return std::strong_ordering::equal; }

All instances of std::monostate compare equal.

The <, <=, >, >=, and != operators are synthesized from operator<=> (since C++20)
and operator== respectively.

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>
#include <cassert>

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;
assert(var.index() == 0);

try {
std::get<S>(var); // throws! We need to assign a value
}
catch(const std::bad_variant_access& e) {
std::cout << e.what() << '\n';
}

var = 12;

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

Possible output:

std::get: wrong index for variant
12