1 // Copyright 2023 The Chromium Authors 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef BASE_TYPES_VARIANT_UTIL_H_ 6 #define BASE_TYPES_VARIANT_UTIL_H_ 7 8 #include <stddef.h> 9 10 #include <type_traits> 11 12 #include "base/types/always_false.h" 13 #include "third_party/abseil-cpp/absl/types/variant.h" 14 15 namespace base { 16 namespace internal { 17 18 template <typename Variant, typename T> 19 struct VariantIndexOfTypeHelper { 20 static_assert(AlwaysFalse<Variant>, "Variant must be an absl::variant<...>"); 21 }; 22 23 template <typename... Ts, typename T> 24 struct VariantIndexOfTypeHelper<absl::variant<Ts...>, T> { 25 static constexpr size_t Index() { 26 static_assert(std::is_constructible_v<absl::variant<LiteralType<Ts>...>, 27 LiteralType<T>>, 28 "Variant is not constructible from T"); 29 return absl::variant<LiteralType<Ts>...>(LiteralType<T>()).index(); 30 } 31 32 // Helper struct; even if `Tag` may not be usable as a literal type, a 33 // `LiteralType<Tag>` will be. 34 template <typename Tag> 35 struct LiteralType {}; 36 }; 37 38 } // namespace internal 39 40 // Returns the 0-based index of `T` in `Variant`'s list of alternative types, 41 // e.g. given `Variant` == `absl::variant<A, B, C>` and `T` == `B`, returns 1. 42 // 43 // Note that this helper cannot be used if the list of alternative types 44 // contains duplicates. 45 template <typename Variant, typename T> 46 constexpr size_t VariantIndexOfType() { 47 return internal::VariantIndexOfTypeHelper<Variant, T>::Index(); 48 } 49 50 } // namespace base 51 52 #endif // BASE_TYPES_VARIANT_UTIL_H_ 53