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std::monostate - cppreference.com

From cppreference.com

Defined in header <variant>

Defined in header <utility>

(since C++26)

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)

(implicitly declared)

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

(destructor)

(implicitly declared)

 trivial implicit destructor
(public member function)

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)
(2)

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; }

 (since C++17)
(until C++20)

constexpr std::strong_ordering operator<=>( monostate, monostate ) noexcept { return std::strong_ordering::equal; }

 (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>;

 (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 output: 

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

See also