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Diffstat (limited to 'third_party/base/template_util.h')
| -rw-r--r-- | third_party/base/template_util.h | 203 |
1 files changed, 203 insertions, 0 deletions
diff --git a/third_party/base/template_util.h b/third_party/base/template_util.h new file mode 100644 index 0000000000..b2463225f8 --- /dev/null +++ b/third_party/base/template_util.h @@ -0,0 +1,203 @@ +// 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 <type_traits> +#include <utility> + +#include "third_party/build/build_config.h" + +// This hacks around libstdc++ 4.6 missing stuff in type_traits, while we need +// to support it. +#define CR_GLIBCXX_4_7_0 20120322 +#define CR_GLIBCXX_4_5_4 20120702 +#define CR_GLIBCXX_4_6_4 20121127 +#if defined(__GLIBCXX__) && \ + (__GLIBCXX__ < CR_GLIBCXX_4_7_0 || __GLIBCXX__ == CR_GLIBCXX_4_5_4 || \ + __GLIBCXX__ == CR_GLIBCXX_4_6_4) +#define CR_USE_FALLBACKS_FOR_OLD_GLIBCXX +#endif + +// 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 {}; + +// is_assignable + +namespace internal { + +template <typename First, typename Second> +struct SelectSecond { + using type = Second; +}; + +struct Any { + Any(...); +}; + +// True case: If |Lvalue| can be assigned to from |Rvalue|, then the return +// value is a true_type. +template <class Lvalue, class Rvalue> +typename internal::SelectSecond< + decltype((std::declval<Lvalue>() = std::declval<Rvalue>())), + std::true_type>::type +IsAssignableTest(Lvalue&&, Rvalue&&); + +// False case: Otherwise the return value is a false_type. +template <class Rvalue> +std::false_type IsAssignableTest(internal::Any, Rvalue&&); + +// Default case: Neither Lvalue nor Rvalue is void. Uses IsAssignableTest to +// determine the type of IsAssignableImpl. +template <class Lvalue, + class Rvalue, + bool = std::is_void<Lvalue>::value || std::is_void<Rvalue>::value> +struct IsAssignableImpl + : public std::common_type<decltype( + internal::IsAssignableTest(std::declval<Lvalue>(), + std::declval<Rvalue>()))>::type {}; + +// Void case: Either Lvalue or Rvalue is void. Then the type of IsAssignableTest +// is false_type. +template <class Lvalue, class Rvalue> +struct IsAssignableImpl<Lvalue, Rvalue, true> : public std::false_type {}; + +// 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 {}; + +} // namespace internal + +// TODO(crbug.com/554293): Remove this when all platforms have this in the std +// namespace. +template <class Lvalue, class Rvalue> +struct is_assignable : public internal::IsAssignableImpl<Lvalue, Rvalue> {}; + +// is_copy_assignable is true if a T const& is assignable to a T&. +// TODO(crbug.com/554293): Remove this when all platforms have this in the std +// namespace. +template <class T> +struct is_copy_assignable + : public is_assignable<typename std::add_lvalue_reference<T>::type, + typename std::add_lvalue_reference< + typename std::add_const<T>::type>::type> {}; + +// is_move_assignable is true if a T&& is assignable to a T&. +// TODO(crbug.com/554293): Remove this when all platforms have this in the std +// namespace. +template <class T> +struct is_move_assignable + : public is_assignable<typename std::add_lvalue_reference<T>::type, + const typename std::add_rvalue_reference<T>::type> { +}; + +// underlying_type produces the integer type backing an enum type. +// TODO(crbug.com/554293): Remove this when all platforms have this in the std +// namespace. +#if defined(CR_USE_FALLBACKS_FOR_OLD_GLIBCXX) +template <typename T> +struct underlying_type { + using type = __underlying_type(T); +}; +#else +template <typename T> +using underlying_type = std::underlying_type<T>; +#endif + +// TODO(crbug.com/554293): Remove this when all platforms have this in the std +// namespace. +#if defined(CR_USE_FALLBACKS_FOR_OLD_GLIBCXX) +template <class T> +using is_trivially_destructible = std::has_trivial_destructor<T>; +#else +template <class T> +using is_trivially_destructible = std::is_trivially_destructible<T>; +#endif + +// 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. In this case we should provide it based on compiler +// intrinsics. This is covered by the CR_USE_FALLBACKS_FOR_OLD_GLIBCXX define. +// - 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_GLIBCXX) || \ + 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); +#endif +}; +#else +template <class T> +using is_trivially_copyable = std::is_trivially_copyable<T>; +#endif + +} // namespace base + +#undef CR_USE_FALLBACKS_FOR_OLD_GLIBCXX +#undef CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX +#undef CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX + +#endif // BASE_TEMPLATE_UTIL_H_ |