Is it possible to wrap a std::holds_alternative to a variadic template to use it with more types?
For example:
std::variant<bool, int, double, std::string> var = 4;
bool r = std::holds_alternative<bool, double>(var); // holds either bool or double
2 Answers
Yes, it's doable with a simple fold expression.
template<typename... Alts, typename... Ts>
constexpr bool holds_any_of(std::variant<Ts...> const& v) noexcept {
return (std::holds_alternative<Alts>(v) || ...);
}
No it's not possible. But you can roll your own:
template <class ...Request, class ...Types>
constexpr bool holdsAlternative(const std::variant<Types...>& v) noexcept
{
return (std::holds_alternative<Request>(v) || ...);
}
to be invoked as you tried it with std::holds_alternative:
bool r = holdsAlternative<bool, double>(var);
