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TMessagesProj/jni/voip/webrtc/base/template_util.h

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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 <vector>
+
+#include "build/build_config.h"
+
+// Some versions of libstdc++ have partial support for type_traits, but misses
+// a smaller subset while removing some of the older non-standard stuff. Assume
+// that all versions below 5.0 fall in this category, along with one 5.0
+// experimental release. Test for this by consulting compiler major version,
+// the only reliable option available, so theoretically this could fail should
+// you attempt to mix an earlier version of libstdc++ with >= GCC5. But
+// that's unlikely to work out, especially as GCC5 changed ABI.
+#define CR_GLIBCXX_5_0_0 20150123
+#if (defined(__GNUC__) && __GNUC__ < 5) || \
+    (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0)
+#define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
+#endif
+
+// This hacks around using gcc with libc++ which has some incompatibilies.
+// - is_trivially_* doesn't work: https://llvm.org/bugs/show_bug.cgi?id=27538
+// TODO(danakj): Remove this when android builders are all using a newer version
+// of gcc, or the android ndk is updated to a newer libc++ that works with older
+// gcc versions.
+#if !defined(__clang__) && defined(_LIBCPP_VERSION)
+#define CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
+#endif
+
+namespace base {
+
+template <class T> struct is_non_const_reference : std::false_type {};
+template <class T> struct is_non_const_reference<T&> : std::true_type {};
+template <class T> struct is_non_const_reference<const T&> : std::false_type {};
+
+namespace internal {
+
+// Implementation detail of base::void_t below.
+template <typename...>
+struct make_void {
+  using type = void;
+};
+
+}  // namespace internal
+
+// base::void_t is an implementation of std::void_t from C++17.
+//
+// We use |base::internal::make_void| as a helper struct to avoid a C++14
+// defect:
+//   http://en.cppreference.com/w/cpp/types/void_t
+//   http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558
+template <typename... Ts>
+using void_t = typename ::base::internal::make_void<Ts...>::type;
+
+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,
+                   void_t<typename std::iterator_traits<T>::iterator_category>>
+    : std::true_type {};
+
+}  // namespace internal
+
+// is_trivially_copyable is especially hard to get right.
+// - Older versions of libstdc++ will fail to have it like they do for other
+//   type traits. This has become a subset of the second point, but used to be
+//   handled independently.
+// - An experimental release of gcc includes most of type_traits but misses
+//   is_trivially_copyable, so we still have to avoid using libstdc++ in this
+//   case, which is covered by CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX.
+// - When compiling libc++ from before r239653, with a gcc compiler, the
+//   std::is_trivially_copyable can fail. So we need to work around that by not
+//   using the one in libc++ in this case. This is covered by the
+//   CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX define, and is discussed in
+//   https://llvm.org/bugs/show_bug.cgi?id=27538#c1 where they point out that
+//   in libc++'s commit r239653 this is fixed by libc++ checking for gcc 5.1.
+// - In both of the above cases we are using the gcc compiler. When defining
+//   this ourselves on compiler intrinsics, the __is_trivially_copyable()
+//   intrinsic is not available on gcc before version 5.1 (see the discussion in
+//   https://llvm.org/bugs/show_bug.cgi?id=27538#c1 again), so we must check for
+//   that version.
+// - When __is_trivially_copyable() is not available because we are on gcc older
+//   than 5.1, we need to fall back to something, so we use __has_trivial_copy()
+//   instead based on what was done one-off in bit_cast() previously.
+
+// TODO(crbug.com/554293): Remove this when all platforms have this in the std
+// namespace and it works with gcc as needed.
+#if defined(CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX) || \
+    defined(CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX)
+template <typename T>
+struct is_trivially_copyable {
+// TODO(danakj): Remove this when android builders are all using a newer version
+// of gcc, or the android ndk is updated to a newer libc++ that does this for
+// us.
+#if _GNUC_VER >= 501
+  static constexpr bool value = __is_trivially_copyable(T);
+#else
+  static constexpr bool value =
+      __has_trivial_copy(T) && __has_trivial_destructor(T);
+#endif
+};
+#else
+template <class T>
+using is_trivially_copyable = std::is_trivially_copyable<T>;
+#endif
+
+#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ <= 7
+// Workaround for g++7 and earlier family.
+// Due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80654, without this
+// Optional<std::vector<T>> where T is non-copyable causes a compile error.
+// As we know it is not trivially copy constructible, explicitly declare so.
+template <typename T>
+struct is_trivially_copy_constructible
+    : std::is_trivially_copy_constructible<T> {};
+
+template <typename... T>
+struct is_trivially_copy_constructible<std::vector<T...>> : std::false_type {};
+#else
+// Otherwise use std::is_trivially_copy_constructible as is.
+template <typename T>
+using is_trivially_copy_constructible = std::is_trivially_copy_constructible<T>;
+#endif
+
+// base::in_place_t is an implementation of std::in_place_t from
+// C++17. A tag type used to request in-place construction in template vararg
+// constructors.
+
+// Specification:
+// https://en.cppreference.com/w/cpp/utility/in_place
+struct in_place_t {};
+constexpr in_place_t in_place = {};
+
+// base::in_place_type_t is an implementation of std::in_place_type_t from
+// C++17. A tag type used for in-place construction when the type to construct
+// needs to be specified, such as with base::unique_any, designed to be a
+// drop-in replacement.
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/in_place
+template <typename T>
+struct in_place_type_t {};
+
+template <typename T>
+struct is_in_place_type_t {
+  static constexpr bool value = false;
+};
+
+template <typename... Ts>
+struct is_in_place_type_t<in_place_type_t<Ts...>> {
+  static constexpr bool value = true;
+};
+
+// C++14 implementation of C++17's std::bool_constant.
+//
+// Reference: https://en.cppreference.com/w/cpp/types/integral_constant
+// Specification: https://wg21.link/meta.type.synop
+template <bool B>
+using bool_constant = std::integral_constant<bool, B>;
+
+// C++14 implementation of C++17's std::conjunction.
+//
+// Reference: https://en.cppreference.com/w/cpp/types/conjunction
+// Specification: https://wg21.link/meta.logical#1.itemdecl:1
+template <typename...>
+struct conjunction : std::true_type {};
+
+template <typename B1>
+struct conjunction<B1> : B1 {};
+
+template <typename B1, typename... Bn>
+struct conjunction<B1, Bn...>
+    : std::conditional_t<static_cast<bool>(B1::value), conjunction<Bn...>, B1> {
+};
+
+// C++14 implementation of C++17's std::disjunction.
+//
+// Reference: https://en.cppreference.com/w/cpp/types/disjunction
+// Specification: https://wg21.link/meta.logical#itemdecl:2
+template <typename...>
+struct disjunction : std::false_type {};
+
+template <typename B1>
+struct disjunction<B1> : B1 {};
+
+template <typename B1, typename... Bn>
+struct disjunction<B1, Bn...>
+    : std::conditional_t<static_cast<bool>(B1::value), B1, disjunction<Bn...>> {
+};
+
+// C++14 implementation of C++17's std::negation.
+//
+// Reference: https://en.cppreference.com/w/cpp/types/negation
+// Specification: https://wg21.link/meta.logical#itemdecl:3
+template <typename B>
+struct negation : bool_constant<!static_cast<bool>(B::value)> {};
+
+}  // namespace base
+
+#undef CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
+#undef CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
+
+#endif  // BASE_TEMPLATE_UTIL_H_