//

// 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>

#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/macros.h"

#include "absl/meta/type_traits.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

// 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::type_identity_t<To> to) {

  return to;

}

// bit_cast()

//

// 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.

//

// 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<int>(f);

//   // i = 0x40490fdb

//

// Reinterpreting and accessing a value directly as a different type (as shown

// below) usually results in undefined behavior.

//

// Example:

//

//   // 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

//

// 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 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) &&

                                std::is_trivially_copyable<Source>::value &&

                                std::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);

}

Unexecuted instantiation: _ZN4absl8bit_castImdTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIjfTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIdmTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIfjTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castItsTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

_ZN4absl8bit_castIstTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

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inline constexpr Dest bit_cast(const Source& source) {

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  return __builtin_bit_cast(Dest, source);

164

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}

Unexecuted instantiation: _ZN4absl8bit_castIjiTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIijTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

_ZN4absl8bit_castImlTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

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inline constexpr Dest bit_cast(const Source& source) {

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  return __builtin_bit_cast(Dest, source);

164

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}

Unexecuted instantiation: _ZN4absl8bit_castIlmTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3std21is_trivially_copyableIS4_EE5valuesr3std21is_trivially_copyableIS3_EE5valueEiE4typeELi0EEES3_RKS4_

#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)

#endif  // defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_BASE_CASTS_H_