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

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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)

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Helper classes

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std::hash<std::monostate>

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

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

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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 variantstd::get: 42std::hash: 0xffffffffffffe19f