std::monostate
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Common vocabulary types
std::variant
(C++26)
monostate
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.
Contents
[edit] Member functions
(constructor)
(implicitly declared)
(public member function)
(destructor)
(implicitly declared)
(public member function)
operator=
(implicitly declared)
(public member function)
[edit] 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; }
(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 true; }
(until C++20)
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<=> and operator== respectively.
[edit] Helper classes
std::hash<std::monostate>
template <>
struct std::hash <monostate>;
(since C++17)
struct std::hash <monostate>;
Specializes the std::hash algorithm for std::monostate.
[edit] Example
Run this code
#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