1 // Copyright (c) 2011 The Chromium Authors. All rights reserved. 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_TEMPLATE_UTIL_H_ 6 #define BASE_TEMPLATE_UTIL_H_ 7 8 #include <stddef.h> 9 #include <iosfwd> 10 #include <iterator> 11 #include <type_traits> 12 #include <utility> 13 #include <vector> 14 15 #include "build/build_config.h" 16 17 // Some versions of libstdc++ have partial support for type_traits, but misses 18 // a smaller subset while removing some of the older non-standard stuff. Assume 19 // that all versions below 5.0 fall in this category, along with one 5.0 20 // experimental release. Test for this by consulting compiler major version, 21 // the only reliable option available, so theoretically this could fail should 22 // you attempt to mix an earlier version of libstdc++ with >= GCC5. But 23 // that's unlikely to work out, especially as GCC5 changed ABI. 24 #define CR_GLIBCXX_5_0_0 20150123 25 #if (defined(__GNUC__) && __GNUC__ < 5) || \ 26 (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0) 27 #define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX 28 #endif 29 30 // This hacks around using gcc with libc++ which has some incompatibilies. 31 // - is_trivially_* doesn't work: https://llvm.org/bugs/show_bug.cgi?id=27538 32 // TODO(danakj): Remove this when android builders are all using a newer version 33 // of gcc, or the android ndk is updated to a newer libc++ that works with older 34 // gcc versions. 35 #if !defined(__clang__) && defined(_LIBCPP_VERSION) 36 #define CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX 37 #endif 38 39 namespace base { 40 41 template <class T> struct is_non_const_reference : std::false_type {}; 42 template <class T> struct is_non_const_reference<T&> : std::true_type {}; 43 template <class T> struct is_non_const_reference<const T&> : std::false_type {}; 44 45 namespace internal { 46 47 // Implementation detail of base::void_t below. 48 template <typename...> 49 struct make_void { 50 using type = void; 51 }; 52 53 } // namespace internal 54 55 // base::void_t is an implementation of std::void_t from C++17. 56 // 57 // We use |base::internal::make_void| as a helper struct to avoid a C++14 58 // defect: 59 // http://en.cppreference.com/w/cpp/types/void_t 60 // http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558 61 template <typename... Ts> 62 using void_t = typename ::base::internal::make_void<Ts...>::type; 63 64 namespace internal { 65 66 // Uses expression SFINAE to detect whether using operator<< would work. 67 template <typename T, typename = void> 68 struct SupportsOstreamOperator : std::false_type {}; 69 template <typename T> 70 struct SupportsOstreamOperator<T, 71 decltype(void(std::declval<std::ostream&>() 72 << std::declval<T>()))> 73 : std::true_type {}; 74 75 // Used to detech whether the given type is an iterator. This is normally used 76 // with std::enable_if to provide disambiguation for functions that take 77 // templatzed iterators as input. 78 template <typename T, typename = void> 79 struct is_iterator : std::false_type {}; 80 81 template <typename T> 82 struct is_iterator<T, 83 void_t<typename std::iterator_traits<T>::iterator_category>> 84 : std::true_type {}; 85 86 } // namespace internal 87 88 // is_trivially_copyable is especially hard to get right. 89 // - Older versions of libstdc++ will fail to have it like they do for other 90 // type traits. This has become a subset of the second point, but used to be 91 // handled independently. 92 // - An experimental release of gcc includes most of type_traits but misses 93 // is_trivially_copyable, so we still have to avoid using libstdc++ in this 94 // case, which is covered by CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX. 95 // - When compiling libc++ from before r239653, with a gcc compiler, the 96 // std::is_trivially_copyable can fail. So we need to work around that by not 97 // using the one in libc++ in this case. This is covered by the 98 // CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX define, and is discussed in 99 // https://llvm.org/bugs/show_bug.cgi?id=27538#c1 where they point out that 100 // in libc++'s commit r239653 this is fixed by libc++ checking for gcc 5.1. 101 // - In both of the above cases we are using the gcc compiler. When defining 102 // this ourselves on compiler intrinsics, the __is_trivially_copyable() 103 // intrinsic is not available on gcc before version 5.1 (see the discussion in 104 // https://llvm.org/bugs/show_bug.cgi?id=27538#c1 again), so we must check for 105 // that version. 106 // - When __is_trivially_copyable() is not available because we are on gcc older 107 // than 5.1, we need to fall back to something, so we use __has_trivial_copy() 108 // instead based on what was done one-off in bit_cast() previously. 109 110 // TODO(crbug.com/554293): Remove this when all platforms have this in the std 111 // namespace and it works with gcc as needed. 112 #if defined(CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX) || \ 113 defined(CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX) 114 template <typename T> 115 struct is_trivially_copyable { 116 // TODO(danakj): Remove this when android builders are all using a newer version 117 // of gcc, or the android ndk is updated to a newer libc++ that does this for 118 // us. 119 #if _GNUC_VER >= 501 120 static constexpr bool value = __is_trivially_copyable(T); 121 #else 122 static constexpr bool value = 123 __has_trivial_copy(T) && __has_trivial_destructor(T); 124 #endif 125 }; 126 #else 127 template <class T> 128 using is_trivially_copyable = std::is_trivially_copyable<T>; 129 #endif 130 131 #if defined(__GNUC__) && !defined(__clang__) && __GNUC__ <= 7 132 // Workaround for g++7 and earlier family. 133 // Due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80654, without this 134 // Optional<std::vector<T>> where T is non-copyable causes a compile error. 135 // As we know it is not trivially copy constructible, explicitly declare so. 136 template <typename T> 137 struct is_trivially_copy_constructible 138 : std::is_trivially_copy_constructible<T> {}; 139 140 template <typename... T> 141 struct is_trivially_copy_constructible<std::vector<T...>> : std::false_type {}; 142 #else 143 // Otherwise use std::is_trivially_copy_constructible as is. 144 template <typename T> 145 using is_trivially_copy_constructible = std::is_trivially_copy_constructible<T>; 146 #endif 147 148 } // namespace base 149 150 #undef CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX 151 #undef CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX 152 153 #endif // BASE_TEMPLATE_UTIL_H_ 154