std::monostate
Defined in header <variant>
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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
constexpr bool operator<(monostate, monostate) noexcept { return false; } constexpr bool operator>(monostate, monostate) noexcept { return false; } |
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All instances of std::monostate
compare equal.
Helper classes
std::hash<std::monostate>
template <> struct std::hash<monostate>; |
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Specializes the std::hash algorithm for std::monostate
.
Example
Run this code
#include <variant> #include <iostream> 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; // var.index() is now 0 - the first element // std::get<S> will throw! We need to assign a value var = 12; std::cout << std::get<S>(var).i << '\n'; }
Output:
12
See also
constructs the variant object (public member function) |