table of contents
std::monostate(3) | C++ Standard Libary | std::monostate(3) |
NAME¶
std::monostate - std::monostate
Synopsis¶
Defined in header <variant>
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 trivial implicit default/copy/move constructor
(implicitly declared) (public member function)
destructor trivial implicit destructor
(implicitly declared) (public member function)
operator= trivial implicit copy/move assignment
(implicitly declared) (public member function)
Non-member functions¶
std::operator==, !=, <, <=, >, >=, <=>(std::monostate)
constexpr bool operator==(monostate, monostate) noexcept { return (since
C++17)
true; }
constexpr bool operator!=(monostate, monostate) noexcept { return
false; }
constexpr bool operator<(monostate, monostate) noexcept { return
false; }
constexpr bool operator>(monostate, monostate) noexcept { return (since
C++17)
false; } (until C++20)
constexpr bool operator<=(monostate, monostate) noexcept { return
true; }
constexpr bool operator>=(monostate, monostate) noexcept { return
true; }
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<=> (since C++20)
and operator== respectively.
Helper classes¶
std::hash<std::monostate>
template <> struct std::hash<monostate>;
Specializes the std::hash algorithm for std::monostate.
Example¶
// Run this code
#include <variant>
#include <iostream>
#include <cassert>
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 = 12;
std::cout << std::get<S>(var).i << '\n';
}
Possible output:¶
std::get: wrong index for variant
12
See also¶
constructor constructs the variant object
(public member function)
2022.07.31 | http://cppreference.com |