diff options
Diffstat (limited to 'absl/base/casts.h')
| -rw-r--r-- | absl/base/casts.h | 129 |
1 files changed, 61 insertions, 68 deletions
diff --git a/absl/base/casts.h b/absl/base/casts.h index 83c69126..b99adb06 100644 --- a/absl/base/casts.h +++ b/absl/base/casts.h @@ -29,6 +29,10 @@ #include <type_traits> #include <utility> +#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L +#include <bit> // For std::bit_cast. +#endif // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L + #include "absl/base/internal/identity.h" #include "absl/base/macros.h" #include "absl/meta/type_traits.h" @@ -36,19 +40,6 @@ namespace absl { ABSL_NAMESPACE_BEGIN -namespace internal_casts { - -template <class Dest, class Source> -struct is_bitcastable - : std::integral_constant< - bool, - sizeof(Dest) == sizeof(Source) && - type_traits_internal::is_trivially_copyable<Source>::value && - type_traits_internal::is_trivially_copyable<Dest>::value && - std::is_default_constructible<Dest>::value> {}; - -} // namespace internal_casts - // implicit_cast() // // Performs an implicit conversion between types following the language @@ -105,81 +96,83 @@ constexpr To implicit_cast(typename absl::internal::identity_t<To> to) { // bit_cast() // -// Performs a bitwise cast on a type without changing the underlying bit -// representation of that type's value. The two types must be of the same size -// and both types must be trivially copyable. As with most casts, use with -// caution. A `bit_cast()` might be needed when you need to temporarily treat a -// type as some other type, such as in the following cases: +// Creates a value of the new type `Dest` whose representation is the same as +// that of the argument, which is of (deduced) type `Source` (a "bitwise cast"; +// every bit in the value representation of the result is equal to the +// corresponding bit in the object representation of the source). Source and +// destination types must be of the same size, and both types must be trivially +// copyable. // -// * Serialization (casting temporarily to `char *` for those purposes is -// always allowed by the C++ standard) -// * Managing the individual bits of a type within mathematical operations -// that are not normally accessible through that type -// * Casting non-pointer types to pointer types (casting the other way is -// allowed by `reinterpret_cast()` but round-trips cannot occur the other -// way). -// -// Example: +// As with most casts, use with caution. A `bit_cast()` might be needed when you +// need to treat a value as the value of some other type, for example, to access +// the individual bits of an object which are not normally accessible through +// the object's type, such as for working with the binary representation of a +// floating point value: // // float f = 3.14159265358979; -// int i = bit_cast<int32_t>(f); +// int i = bit_cast<int>(f); // // i = 0x40490fdb // -// Casting non-pointer types to pointer types and then dereferencing them -// traditionally produces undefined behavior. +// Reinterpreting and accessing a value directly as a different type (as shown +// below) usually results in undefined behavior. // // Example: // // // WRONG -// float f = 3.14159265358979; // WRONG -// int i = * reinterpret_cast<int*>(&f); // WRONG +// float f = 3.14159265358979; +// int i = reinterpret_cast<int&>(f); // Wrong +// int j = *reinterpret_cast<int*>(&f); // Equally wrong +// int k = *bit_cast<int*>(&f); // Equally wrong // -// The address-casting method produces undefined behavior according to the ISO -// C++ specification section [basic.lval]. Roughly, this section says: if an -// object in memory has one type, and a program accesses it with a different -// type, the result is undefined behavior for most values of "different type". +// Reinterpret-casting results in undefined behavior according to the ISO C++ +// specification, section [basic.lval]. Roughly, this section says: if an object +// in memory has one type, and a program accesses it with a different type, the +// result is undefined behavior for most "different type". +// +// Using bit_cast on a pointer and then dereferencing it is no better than using +// reinterpret_cast. You should only use bit_cast on the value itself. // // Such casting results in type punning: holding an object in memory of one type // and reading its bits back using a different type. A `bit_cast()` avoids this -// issue by implementing its casts using `memcpy()`, which avoids introducing -// this undefined behavior. -// -// NOTE: The requirements here are more strict than the bit_cast of standard -// proposal p0476 due to the need for workarounds and lack of intrinsics. -// Specifically, this implementation also requires `Dest` to be -// default-constructible. -template < - typename Dest, typename Source, - typename std::enable_if<internal_casts::is_bitcastable<Dest, Source>::value, - int>::type = 0> +// issue by copying the object representation to a new value, which avoids +// introducing this undefined behavior (since the original value is never +// accessed in the wrong way). +// +// The requirements of `absl::bit_cast` are more strict than that of +// `std::bit_cast` unless compiler support is available. Specifically, without +// compiler support, this implementation also requires `Dest` to be +// default-constructible. In C++20, `absl::bit_cast` is replaced by +// `std::bit_cast`. +#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L + +using std::bit_cast; + +#else // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L + +template <typename Dest, typename Source, + typename std::enable_if< + sizeof(Dest) == sizeof(Source) && + type_traits_internal::is_trivially_copyable<Source>::value && + type_traits_internal::is_trivially_copyable<Dest>::value +#if !ABSL_HAVE_BUILTIN(__builtin_bit_cast) + && std::is_default_constructible<Dest>::value +#endif // !ABSL_HAVE_BUILTIN(__builtin_bit_cast) + , + int>::type = 0> +#if ABSL_HAVE_BUILTIN(__builtin_bit_cast) +inline constexpr Dest bit_cast(const Source& source) { + return __builtin_bit_cast(Dest, source); +} +#else // ABSL_HAVE_BUILTIN(__builtin_bit_cast) inline Dest bit_cast(const Source& source) { Dest dest; memcpy(static_cast<void*>(std::addressof(dest)), static_cast<const void*>(std::addressof(source)), sizeof(dest)); return dest; } +#endif // ABSL_HAVE_BUILTIN(__builtin_bit_cast) -// NOTE: This overload is only picked if the requirements of bit_cast are -// not met. It is therefore UB, but is provided temporarily as previous -// versions of this function template were unchecked. Do not use this in -// new code. -template < - typename Dest, typename Source, - typename std::enable_if< - !internal_casts::is_bitcastable<Dest, Source>::value, - int>::type = 0> -ABSL_DEPRECATED( - "absl::bit_cast type requirements were violated. Update the types " - "being used such that they are the same size and are both " - "TriviallyCopyable.") -inline Dest bit_cast(const Source& source) { - static_assert(sizeof(Dest) == sizeof(Source), - "Source and destination types should have equal sizes."); - - Dest dest; - memcpy(&dest, &source, sizeof(dest)); - return dest; -} +#endif // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L ABSL_NAMESPACE_END } // namespace absl |