std::hashstd::variant - cppreference.com (original) (raw)

| | | | | ------------------------------------------------------------------------------------ | | ------------- | | 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

[edit] Notes

Unlike std::hashstd::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

#include #include #include   using Var = std::variant<int, int, int, std::string>;   template void print(Var const& var) { std::cout << "get<" << var.index() << "> = " << std::get(var) << "\t" "# = " << std::hash{}(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

| | hash function object (class template) [edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |