diff options
Diffstat (limited to 'third_party/abseil_cpp/absl/base/casts.h')
| -rw-r--r-- | third_party/abseil_cpp/absl/base/casts.h | 187 |
1 files changed, 187 insertions, 0 deletions
diff --git a/third_party/abseil_cpp/absl/base/casts.h b/third_party/abseil_cpp/absl/base/casts.h new file mode 100644 index 000000000000..83c691265ff3 --- /dev/null +++ b/third_party/abseil_cpp/absl/base/casts.h @@ -0,0 +1,187 @@ +// +// Copyright 2017 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. +// +// ----------------------------------------------------------------------------- +// File: casts.h +// ----------------------------------------------------------------------------- +// +// This header file defines casting templates to fit use cases not covered by +// the standard casts provided in the C++ standard. As with all cast operations, +// use these with caution and only if alternatives do not exist. + +#ifndef ABSL_BASE_CASTS_H_ +#define ABSL_BASE_CASTS_H_ + +#include <cstring> +#include <memory> +#include <type_traits> +#include <utility> + +#include "absl/base/internal/identity.h" +#include "absl/base/macros.h" +#include "absl/meta/type_traits.h" + +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 +// rules for implicit conversion; if an implicit conversion is otherwise +// allowed by the language in the given context, this function performs such an +// implicit conversion. +// +// Example: +// +// // If the context allows implicit conversion: +// From from; +// To to = from; +// +// // Such code can be replaced by: +// implicit_cast<To>(from); +// +// An `implicit_cast()` may also be used to annotate numeric type conversions +// that, although safe, may produce compiler warnings (such as `long` to `int`). +// Additionally, an `implicit_cast()` is also useful within return statements to +// indicate a specific implicit conversion is being undertaken. +// +// Example: +// +// return implicit_cast<double>(size_in_bytes) / capacity_; +// +// Annotating code with `implicit_cast()` allows you to explicitly select +// particular overloads and template instantiations, while providing a safer +// cast than `reinterpret_cast()` or `static_cast()`. +// +// Additionally, an `implicit_cast()` can be used to allow upcasting within a +// type hierarchy where incorrect use of `static_cast()` could accidentally +// allow downcasting. +// +// Finally, an `implicit_cast()` can be used to perform implicit conversions +// from unrelated types that otherwise couldn't be implicitly cast directly; +// C++ will normally only implicitly cast "one step" in such conversions. +// +// That is, if C is a type which can be implicitly converted to B, with B being +// a type that can be implicitly converted to A, an `implicit_cast()` can be +// used to convert C to B (which the compiler can then implicitly convert to A +// using language rules). +// +// Example: +// +// // Assume an object C is convertible to B, which is implicitly convertible +// // to A +// A a = implicit_cast<B>(C); +// +// Such implicit cast chaining may be useful within template logic. +template <typename To> +constexpr To implicit_cast(typename absl::internal::identity_t<To> to) { + return 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: +// +// * 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: +// +// float f = 3.14159265358979; +// int i = bit_cast<int32_t>(f); +// // i = 0x40490fdb +// +// Casting non-pointer types to pointer types and then dereferencing them +// traditionally produces undefined behavior. +// +// Example: +// +// // WRONG +// float f = 3.14159265358979; // WRONG +// int i = * reinterpret_cast<int*>(&f); // 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". +// +// 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> +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; +} + +// 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; +} + +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_BASE_CASTS_H_ |