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(C++26) | ||||
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hash<std::variant> | ||||
| Helper objects | ||||
Defined in header <variant> | ||
template<class...Types> struct hash<std::variant<Types...>>; | (since C++17) | |
The template specialization ofstd::hash for thestd::variant template allows users to obtain hashes ofvariant objects.
The specializationstd::hash<std::variant<Types...>> is enabled (seestd::hash) if every specialization instd::hash<std::remove_const_t<Types>>... is enabled, and is disabled otherwise.
The member functions of this specialization are not guaranteed to be noexcept.
Contents |
| Types | - | the types of the alternatives supported by thevariant object |
Unlikestd::hash<std::optional>, hash of a variant does not typically equal the hash of the contained value; this makes it possible to distinguishstd::variant<int,int> holding the same value as different alternatives.
#include <iostream>#include <string>#include <variant> using Var=std::variant<int,int,int,std::string>; template<unsigned I>void print(Varconst& var){std::cout<<"get<"<< var.index()<<"> = "<< std::get<I>(var)<<"\t""# = "<<std::hash<Var>{}(var)<<'\n';} int main(){ Var var; std::get<0>(var)=2020; print<0>(var); var.emplace<1>(2023); print<1>(var); var.emplace<2>(2026); print<2>(var); var="C++"; print<3>(var);}
Possible output:
get<0> = 2020 # = 2020get<1> = 2023 # = 2024get<2> = 2026 # = 2028get<3> = C++ # = 15518724754199266859
(C++11) | hash function object (class template)[edit] |
