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- // Copyright (c) 2011 The Chromium Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- #ifndef BASE_TEMPLATE_UTIL_H_
- #define BASE_TEMPLATE_UTIL_H_
- #include <stddef.h>
- #include <iosfwd>
- #include <iterator>
- #include <type_traits>
- #include <utility>
- #include "base/compiler_specific.h"
- namespace base {
- namespace internal {
- // Uses expression SFINAE to detect whether using operator<< would work.
- template <typename T, typename = void>
- struct SupportsOstreamOperator : std::false_type {};
- template <typename T>
- struct SupportsOstreamOperator<T,
- decltype(void(std::declval<std::ostream&>()
- << std::declval<T>()))>
- : std::true_type {};
- template <typename T, typename = void>
- struct SupportsToString : std::false_type {};
- template <typename T>
- struct SupportsToString<T, decltype(void(std::declval<T>().ToString()))>
- : std::true_type {};
- // Used to detech whether the given type is an iterator. This is normally used
- // with std::enable_if to provide disambiguation for functions that take
- // templatzed iterators as input.
- template <typename T, typename = void>
- struct is_iterator : std::false_type {};
- template <typename T>
- struct is_iterator<
- T,
- std::void_t<typename std::iterator_traits<T>::iterator_category>>
- : std::true_type {};
- // Helper to express preferences in an overload set. If more than one overload
- // are available for a given set of parameters the overload with the higher
- // priority will be chosen.
- template <size_t I>
- struct priority_tag : priority_tag<I - 1> {};
- template <>
- struct priority_tag<0> {};
- } // namespace internal
- namespace internal {
- // The indirection with std::is_enum<T> is required, because instantiating
- // std::underlying_type_t<T> when T is not an enum is UB prior to C++20.
- template <typename T, bool = std::is_enum<T>::value>
- struct IsScopedEnumImpl : std::false_type {};
- template <typename T>
- struct IsScopedEnumImpl<T, /*std::is_enum<T>::value=*/true>
- : std::negation<std::is_convertible<T, std::underlying_type_t<T>>> {};
- } // namespace internal
- // Implementation of C++23's std::is_scoped_enum
- //
- // Reference: https://en.cppreference.com/w/cpp/types/is_scoped_enum
- template <typename T>
- struct is_scoped_enum : internal::IsScopedEnumImpl<T> {};
- // Implementation of C++20's std::remove_cvref.
- //
- // References:
- // - https://en.cppreference.com/w/cpp/types/remove_cvref
- // - https://wg21.link/meta.trans.other#lib:remove_cvref
- template <typename T>
- struct remove_cvref {
- using type = std::remove_cv_t<std::remove_reference_t<T>>;
- };
- // Implementation of C++20's std::remove_cvref_t.
- //
- // References:
- // - https://en.cppreference.com/w/cpp/types/remove_cvref
- // - https://wg21.link/meta.type.synop#lib:remove_cvref_t
- template <typename T>
- using remove_cvref_t = typename remove_cvref<T>::type;
- // Implementation of C++20's std::is_constant_evaluated.
- //
- // References:
- // - https://en.cppreference.com/w/cpp/types/is_constant_evaluated
- // - https://wg21.link/meta.const.eval
- constexpr bool is_constant_evaluated() noexcept {
- #if HAS_BUILTIN(__builtin_is_constant_evaluated)
- return __builtin_is_constant_evaluated();
- #else
- return false;
- #endif
- }
- // Simplified implementation of C++20's std::iter_value_t.
- // As opposed to std::iter_value_t, this implementation does not restrict
- // the type of `Iter` and does not consider specializations of
- // `indirectly_readable_traits`.
- //
- // Reference: https://wg21.link/readable.traits#2
- template <typename Iter>
- struct IterValueImpl {
- using value_type = typename std::iterator_traits<Iter>::value_type;
- };
- template <typename T, bool Cond = false>
- struct IterValuePointerImpl {
- // The `iterator_traits<T*>::value_type` member is not defined if T is not an
- // object in C++20.
- };
- template <typename T>
- struct IterValuePointerImpl<T*, true> {
- using value_type = typename std::iterator_traits<T*>::value_type;
- };
- template <typename T>
- struct IterValueImpl<T*> {
- using value_type =
- typename IterValuePointerImpl<T*, std::is_object_v<T>>::value_type;
- };
- template <typename Iter>
- using iter_value_t = typename IterValueImpl<remove_cvref_t<Iter>>::value_type;
- // Simplified implementation of C++20's std::iter_reference_t.
- // As opposed to std::iter_reference_t, this implementation does not restrict
- // the type of `Iter`.
- //
- // Reference: https://wg21.link/iterator.synopsis#:~:text=iter_reference_t
- template <typename Iter>
- using iter_reference_t = decltype(*std::declval<Iter&>());
- // Simplified implementation of C++20's std::indirect_result_t. As opposed to
- // std::indirect_result_t, this implementation does not restrict the type of
- // `Func` and `Iters`.
- //
- // Reference: https://wg21.link/iterator.synopsis#:~:text=indirect_result_t
- template <typename Func, typename... Iters>
- using indirect_result_t =
- std::invoke_result_t<Func, iter_reference_t<Iters>...>;
- // Simplified implementation of C++20's std::projected. As opposed to
- // std::projected, this implementation does not explicitly restrict the type of
- // `Iter` and `Proj`, but rather does so implicitly by requiring
- // `indirect_result_t<Proj, Iter>` is a valid type. This is required for SFINAE
- // friendliness.
- //
- // Reference: https://wg21.link/projected
- template <typename Iter,
- typename Proj,
- typename IndirectResultT = indirect_result_t<Proj, Iter>>
- struct projected {
- using value_type = remove_cvref_t<IndirectResultT>;
- IndirectResultT operator*() const; // not defined
- };
- } // namespace base
- #endif // BASE_TEMPLATE_UTIL_H_
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