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std::hash<std::variant>

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Defined in header <variant>
template< class... Types >
struct hash<std::variant <Types...>>;
(since C++17)

The template specialization of std::hash for the std::variant template allows users to obtain hashes of variant objects.

The specialization std::hash<std::variant <Types...>> is enabled (see std::hash ) if every specialization in std::hash<std::remove_const_t <Types>>... is enabled, and is disabled otherwise.

The member functions of this specialization are not guaranteed to be noexcept.

[edit] Template parameters

Types - the types of the alternatives supported by the variant object

[edit] Notes

Unlike std::hash<std::optional>, hash of a variant does not typically equal the hash of the contained value; this makes it possible to distinguish std::variant <int, int> holding the same value as different alternatives.

[edit] Example

Run this code
#include <iostream>
#include <string>
#include <variant>
 
using Var = std::variant <int, int, int, std::string >;
 
template<unsigned I>
void print(Var const& 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 # = 2020
get<1> = 2023 # = 2024
get<2> = 2026 # = 2028
get<3> = C++ # = 15518724754199266859

[edit] See also

(C++11)
hash function object
(class template) [edit]
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