| //===----------------------------------------------------------------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #ifndef _LIBCPP___TYPE_TRAITS_IS_ALWAYS_BITCASTABLE_H |
| #define _LIBCPP___TYPE_TRAITS_IS_ALWAYS_BITCASTABLE_H |
| |
| #include <__config> |
| #include <__type_traits/integral_constant.h> |
| #include <__type_traits/is_integral.h> |
| #include <__type_traits/is_object.h> |
| #include <__type_traits/is_same.h> |
| #include <__type_traits/is_trivially_copyable.h> |
| #include <__type_traits/remove_cv.h> |
| |
| #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) |
| # pragma GCC system_header |
| #endif |
| |
| _LIBCPP_BEGIN_NAMESPACE_STD |
| |
| // Checks whether an object of type `From` can always be bit-cast to an object of type `To` and represent a valid value |
| // of type `To`. In other words, `From` and `To` have the same value representation and the set of values of `From` is |
| // a subset of the set of values of `To`. |
| // |
| // Note that types that cannot be assigned to each other using built-in assignment (e.g. arrays) might still be |
| // considered bit-castable. |
| template <class _From, class _To> |
| struct __is_always_bitcastable { |
| using _UnqualFrom = __remove_cv_t<_From>; |
| using _UnqualTo = __remove_cv_t<_To>; |
| |
| // clang-format off |
| static const bool value = |
| // First, the simple case -- `From` and `To` are the same object type. |
| (is_same<_UnqualFrom, _UnqualTo>::value && is_trivially_copyable<_UnqualFrom>::value) || |
| |
| // Beyond the simple case, we say that one type is "always bit-castable" to another if: |
| // - (1) `From` and `To` have the same value representation, and in addition every possible value of `From` has |
| // a corresponding value in the `To` type (in other words, the set of values of `To` is a superset of the set of |
| // values of `From`); |
| // - (2) When the corresponding values are not the same value (as, for example, between an unsigned and a signed |
| // integer, where a large positive value of the unsigned integer corresponds to a negative value in the signed |
| // integer type), the value of `To` that results from a bitwise copy of `From` is the same what would be |
| // produced by the built-in assignment (if it were defined for the two types, to which there are minor |
| // exceptions, e.g. built-in arrays). |
| // |
| // In practice, that means: |
| // - all integral types (except `bool`, see below) -- that is, character types and `int` types, both signed and |
| // unsigned... |
| // - as well as arrays of such types... |
| // - ...that have the same size. |
| // |
| // Other trivially-copyable types can't be validly bit-cast outside of their own type: |
| // - floating-point types normally have different sizes and thus aren't bit-castable between each other (fails |
| // #1); |
| // - integral types and floating-point types use different representations, so for example bit-casting an integral |
| // `1` to `float` results in a very small less-than-one value, unlike built-in assignment that produces `1.0` |
| // (fails #2); |
| // - booleans normally use only a single bit of their object representation; bit-casting an integer to a boolean |
| // will result in a boolean object with an incorrect representation, which is undefined behavior (fails #2). |
| // Bit-casting from a boolean into an integer, however, is valid; |
| // - enumeration types may have different ranges of possible values (fails #1); |
| // - for pointers, it is not guaranteed that pointers to different types use the same set of values to represent |
| // addresses, and the conversion results are explicitly unspecified for types with different alignments |
| // (fails #1); |
| // - for structs and unions it is impossible to determine whether the set of values of one of them is a subset of |
| // the other (fails #1); |
| // - there is no need to consider `nullptr_t` for practical purposes. |
| ( |
| sizeof(_From) == sizeof(_To) && |
| is_integral<_From>::value && |
| is_integral<_To>::value && |
| !is_same<_UnqualTo, bool>::value |
| ); |
| // clang-format on |
| }; |
| |
| _LIBCPP_END_NAMESPACE_STD |
| |
| #endif // _LIBCPP___TYPE_TRAITS_IS_ALWAYS_BITCASTABLE_H |