std:: hash <std::variant>

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.

Template parameters

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.

Example

#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);
}

get<0> = 2020   # = 2020
get<1> = 2023   # = 2024
get<2> = 2026   # = 2028
get<3> = C++    # = 15518724754199266859

See also