std::monostate

Defined in header <variant>.

C++17

struct monostate { };

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

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

Non-member functions

---

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

constexpr bool operator==( monostate, monostate ) noexcept { return true; }     // 1) (since C++17)

constexpr bool operator!=( monostate, monostate ) noexcept { return false; }
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; }     // 2) (C++17)

constexpr std::strong_ordering operator<=>( monostate, monostate ) noexcept     
{
    return std::strong_ordering::equal;
}                                                                               // 2) (since C++20)

All instances of std::monostate compare equal.

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

---

Helper classes

---

std::hash<std::monostate>

template <>
struct std::hash<monostate>;    // 2) (since C++17) 

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

---

Example

#include <cassert>
#include <iostream>
#include <variant>
 
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 = 42;
    std::cout 
        << "std::get: " 
        << std::get<S>(var).i << '\n'
        << "std::hash: " 
        << std::hex << std::showbase
        << std::hash<std::monostate>{}(std::monostate{})
         << '\n';
}

Possible Result

std::get: wrong index for variant
std::get: 42
std::hash: 0xffffffffffffe19f