//
// 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 "absl/base/internal/identity.h"
#include "absl/base/macros.h"
namespace absl {
namespace internal_casts {
// NOTE: Not a fully compliant implementation of `std::is_trivially_copyable`.
// TODO(calabrese) Branch on implementations that directly provide
// `std::is_trivially_copyable`, create a more rigorous workaround, and publicly
// expose in meta/type_traits.
template <class T>
struct is_trivially_copyable
: std::integral_constant<
bool, std::is_destructible<T>::value&& __has_trivial_destructor(T) &&
__has_trivial_copy(T) && __has_trivial_assign(T)> {};
template <class Dest, class Source>
struct is_bitcastable
: std::integral_constant<bool,
sizeof(Dest) == sizeof(Source) &&
is_trivially_copyable<Source>::value &&
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;
}
} // namespace absl
#endif // ABSL_BASE_CASTS_H_
