//

// 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 <typeinfo>

#include <utility>

#if __has_include(<version>)

#include <version>  // For __cpp_lib_bit_cast.

#endif

#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/attributes.h"

#include "absl/base/config.h"

#include "absl/base/macros.h"

#include "absl/base/optimization.h"

#include "absl/base/options.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 std::enable_if_t<

    !type_traits_internal::IsView<std::enable_if_t<

        !std::is_reference_v<To>, std::remove_cv_t<To>>>::value,

    To>

implicit_cast(absl::type_identity_t<To> to) {

  return to;

}

template <typename To>

constexpr std::enable_if_t<

    type_traits_internal::IsView<std::enable_if_t<!std::is_reference_v<To>,

                                                  std::remove_cv_t<To>>>::value,

    To>

implicit_cast(absl::type_identity_t<To> to ABSL_ATTRIBUTE_LIFETIME_BOUND) {

  return to;

}

template <typename To>

constexpr std::enable_if_t<std::is_reference_v<To>, To> implicit_cast(

    absl::type_identity_t<To> to ABSL_ATTRIBUTE_LIFETIME_BOUND) {

  return std::forward<absl::type_identity_t<To>>(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,

          std::enable_if_t<sizeof(Dest) == sizeof(Source) &&

                               std::is_trivially_copyable_v<Source> &&

                               std::is_trivially_copyable_v<Dest>

#if !ABSL_HAVE_BUILTIN(__builtin_bit_cast)

                               && std::is_default_constructible_v<Dest>

#endif  // !ABSL_HAVE_BUILTIN(__builtin_bit_cast)

                           ,

                           int> = 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_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIjfTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIdmTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIfjTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castItsTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIstTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIjiTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIijTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castImlTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_RKS4_

Unexecuted instantiation: _ZN4absl8bit_castIlmTnNSt3__19enable_ifIXaaaaeqstT_stT0_sr3stdE23is_trivially_copyable_vIS4_Esr3stdE23is_trivially_copyable_vIS3_EEiE4typeELi0EEES3_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

namespace base_internal {

[[noreturn]] ABSL_ATTRIBUTE_NOINLINE void BadDownCastCrash(

    const char* source_type, const char* target_type);

template <typename To, typename From>

inline void ValidateDownCast(From* f) {

  // Assert only if RTTI is enabled and in debug mode or hardened asserts are

  // enabled.

#if defined(ABSL_INTERNAL_HAS_RTTI) && \

    (!defined(NDEBUG) || (ABSL_OPTION_HARDENED == 1))

  // Suppress erroneous nonnull comparison warning on older GCC.

#if defined(__GNUC__) && !defined(__clang__)

#pragma GCC diagnostic push

#pragma GCC diagnostic ignored "-Wnonnull-compare"

#endif

  if (ABSL_PREDICT_FALSE(f != nullptr && dynamic_cast<To>(f) == nullptr)) {

#if defined(__GNUC__) && !defined(__clang__)

#pragma GCC diagnostic pop

#endif

    absl::base_internal::BadDownCastCrash(

        typeid(*f).name(), typeid(std::remove_pointer_t<To>).name());

  }

#else

  (void)f;  // Denote this as "used" to avoid warning

#endif

}

}  // namespace base_internal

// An "upcast", i.e. a conversion from a pointer to an object to a pointer to a

// base subobject, always succeeds if the base is unambiguous and accessible,

// and so it's fine to use implicit_cast.

//

// A "downcast", i.e. a conversion from a pointer to an object to a pointer

// to a more-derived object that may contain the original object as a base

// subobject, cannot safely be done using static_cast, because you do not

// generally know whether the source object is really the base subobject of

// a containing, more-derived object of the target type. Thus, when you

// downcast in a polymorphic type hierarchy, you should use the following

// function template.

//

// This function only returns null when the input is null. In debug mode, we

// use dynamic_cast to double-check whether the downcast is legal (we die if

// it's not). In normal mode, we do the efficient static_cast instead. Because

// the process will die in debug mode, it's important to test to make sure the

// cast is legal before calling this function!

//

// dynamic_cast should be avoided except as allowed by the style guide

// (https://google.github.io/styleguide/cppguide.html#Run-Time_Type_Information__RTTI_).

template <typename To, typename From>  // use like this: down_cast<T*>(foo);

[[nodiscard]]

inline To down_cast(From* f) {  // so we only accept pointers

  static_assert(std::is_pointer_v<To>, "target type not a pointer");

  // dynamic_cast allows casting to the same type or a more cv-qualified

  // version of the same type without them being polymorphic.

  if constexpr (!std::is_same_v<std::remove_cv_t<std::remove_pointer_t<To>>,

                                std::remove_cv_t<From>>) {

    static_assert(std::is_polymorphic_v<From>,

                  "source type must be polymorphic");

    static_assert(std::is_polymorphic_v<std::remove_pointer_t<To>>,

                  "target type must be polymorphic");

  }

  static_assert(

      std::is_convertible_v<std::remove_cv_t<std::remove_pointer_t<To>>*,

                            std::remove_cv_t<From>*>,

      "target type not derived from source type");

  absl::base_internal::ValidateDownCast<To>(f);

  return static_cast<To>(f);

}

// Overload of down_cast for references. Use like this:

// absl::down_cast<T&>(foo). The code is slightly convoluted because we're still

// using the pointer form of dynamic cast. (The reference form throws an

// exception if it fails.)

//

// There's no need for a special const overload either for the pointer

// or the reference form. If you call down_cast with a const T&, the

// compiler will just bind From to const T.

template <typename To, typename From>

[[nodiscard]]

inline To down_cast(From& f) {

  static_assert(std::is_lvalue_reference_v<To>, "target type not a reference");

  // dynamic_cast allows casting to the same type or a more cv-qualified

  // version of the same type without them being polymorphic.

  if constexpr (!std::is_same_v<std::remove_cv_t<std::remove_reference_t<To>>,

                                std::remove_cv_t<From>>) {

    static_assert(std::is_polymorphic_v<From>,

                  "source type must be polymorphic");

    static_assert(std::is_polymorphic_v<std::remove_reference_t<To>>,

                  "target type must be polymorphic");

  }

  static_assert(

      std::is_convertible_v<std::remove_cv_t<std::remove_reference_t<To>>*,

                            std::remove_cv_t<From>*>,

      "target type not derived from source type");

  absl::base_internal::ValidateDownCast<std::remove_reference_t<To>*>(

      std::addressof(f));

  return static_cast<To>(f);

}

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_BASE_CASTS_H_