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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 instd::variant. In particular, a variant of non-default-constructible types may liststd::monostate as its first alternative: this makes the variant itself default-constructible.
Contents |
(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) |
constexprbool operator==( monostate, monostate)noexcept{returntrue;} | (1) | (since C++17) |
| (2) | ||
constexprbool operator!=( monostate, monostate)noexcept{returnfalse;} constexprbool operator<( monostate, monostate)noexcept{returnfalse;} | (since C++17) (until C++20) | |
constexprstd::strong_ordering operator<=>( monostate, monostate)noexcept { | (since C++20) | |
All instances ofstd::monostate compare equal.
The | (since C++20) |
template<> structstd::hash<monostate>; | (since C++17) | |
Specializes thestd::hash algorithm forstd::monostate.
#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(conststd::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 variantstd::get: 42std::hash: 0xffffffffffffe19f
constructs thevariant object(public member function)[edit] |
