1 //===----------------------------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 #ifndef _LIBCPP___TYPE_TRAITS_DATASIZEOF_H 10 #define _LIBCPP___TYPE_TRAITS_DATASIZEOF_H 11 12 #include <__config> 13 #include <__type_traits/is_class.h> 14 #include <__type_traits/is_final.h> 15 #include <cstddef> 16 17 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 18 # pragma GCC system_header 19 #endif 20 21 // This trait provides the size of a type excluding any tail padding. 22 // 23 // It is useful in contexts where performing an operation using the full size of the class (including padding) may 24 // have unintended side effects, such as overwriting a derived class' member when writing the tail padding of a class 25 // through a pointer-to-base. 26 27 _LIBCPP_BEGIN_NAMESPACE_STD 28 29 #if __has_keyword(__datasizeof) || __has_extension(datasizeof) 30 template <class _Tp> 31 inline const size_t __datasizeof_v = __datasizeof(_Tp); 32 #else 33 // NOLINTNEXTLINE(readability-redundant-preprocessor) This is https://llvm.org/PR64825 34 # if __has_cpp_attribute(__no_unique_address__) 35 template <class _Tp> 36 struct _FirstPaddingByte { 37 [[__no_unique_address__]] _Tp __v_; 38 char __first_padding_byte_; 39 }; 40 # else 41 template <class _Tp, bool = __libcpp_is_final<_Tp>::value || !is_class<_Tp>::value> 42 struct _FirstPaddingByte : _Tp { 43 char __first_padding_byte_; 44 }; 45 46 template <class _Tp> 47 struct _FirstPaddingByte<_Tp, true> { 48 _Tp __v_; 49 char __first_padding_byte_; 50 }; 51 # endif // __has_cpp_attribute(__no_unique_address__) 52 53 // _FirstPaddingByte<> is sometimes non-standard layout. Using `offsetof` is UB in that case, but GCC and Clang allow 54 // the use as an extension. 55 _LIBCPP_DIAGNOSTIC_PUSH 56 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-offsetof") 57 _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Winvalid-offsetof") 58 template <class _Tp> 59 inline const size_t __datasizeof_v = offsetof(_FirstPaddingByte<_Tp>, __first_padding_byte_); 60 _LIBCPP_DIAGNOSTIC_POP 61 #endif // __has_extension(datasizeof) 62 63 _LIBCPP_END_NAMESPACE_STD 64 65 #endif // _LIBCPP___TYPE_TRAITS_DATASIZEOF_H 66