libartbase/base/casts.h - platform/art - Git at Google

 /*
  * Copyright (C) 2011 The Android Open Source Project
  *
  * 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
  *
  *      http://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.
  */

 #ifndef ART_LIBARTBASE_BASE_CASTS_H_
 #define ART_LIBARTBASE_BASE_CASTS_H_

 #include <assert.h>
 #include <stdint.h>
 #include <string.h>

 #include <limits>
 #include <type_traits>

 #include <android-base/logging.h>

 #include "stl_util_identity.h"

 namespace art {

 // Use implicit_cast as a safe version of static_cast or const_cast
 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo
 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to
 // a const pointer to Foo).
 // When you use implicit_cast, the compiler checks that the cast is safe.
 // Such explicit implicit_casts are necessary in surprisingly many
 // situations where C++ demands an exact type match instead of an
 // argument type convertible to a target type.
 //
 // The From type can be inferred, so the preferred syntax for using
 // implicit_cast is the same as for static_cast etc.:
 //
 //   implicit_cast<ToType>(expr)
 //
 // implicit_cast would have been part of the C++ standard library,
 // but the proposal was submitted too late.  It will probably make
 // its way into the language in the future.
 template<typename To, typename From>
 inline To implicit_cast(From const &f) {
   return f;
 }

 // When you upcast (that is, cast a pointer from type Foo to type
 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
 // always succeed.  When you downcast (that is, cast a pointer from
 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
 // how do you know the pointer is really of type SubclassOfFoo?  It
 // could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
 // when you downcast, you should use this macro.  In debug mode, we
 // use dynamic_cast<> to double-check the downcast is legal (we die
 // if it's not).  In normal mode, we do the efficient static_cast<>
 // instead.  Thus, it's important to test in debug mode to make sure
 // the cast is legal!
 //    This is the only place in the code we should use dynamic_cast<>.
 // In particular, you SHOULDN'T be using dynamic_cast<> in order to
 // do RTTI (eg code like this:
 //    if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
 //    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
 // You should design the code some other way not to need this.

 template<typename To, typename From>     // use like this: down_cast<T*>(foo);
 inline To down_cast(From* f) {                   // so we only accept pointers
   static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value,
                 "down_cast unsafe as To is not a subtype of From");

   return static_cast<To>(f);
 }

 template<typename To, typename From>     // use like this: down_cast<T&>(foo);
 inline To down_cast(From& f) {           // so we only accept references
   static_assert(std::is_base_of<From, typename std::remove_reference<To>::type>::value,
                 "down_cast unsafe as To is not a subtype of From");

   return static_cast<To>(f);
 }

 template <class Dest, class Source>
 inline Dest bit_cast(const Source& source) {
   // Compile time assertion: sizeof(Dest) == sizeof(Source)
   // A compile error here means your Dest and Source have different sizes.
   static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal");
   Dest dest;
   memcpy(&dest, &source, sizeof(dest));
   return dest;
 }

 // A version of static_cast that DCHECKs that the value can be precisely represented
 // when converting to Dest.
 template <typename Dest, typename Source>
 constexpr Dest dchecked_integral_cast(Source source) {
   DCHECK(
       // Check that the value is within the lower limit of Dest.
       (static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <=
           static_cast<intmax_t>(std::numeric_limits<Source>::min()) ||
           source >= static_cast<Source>(std::numeric_limits<Dest>::min())) &&
       // Check that the value is within the upper limit of Dest.
       (static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >=
           static_cast<uintmax_t>(std::numeric_limits<Source>::max()) ||
           source <= static_cast<Source>(std::numeric_limits<Dest>::max())))
       << "dchecked_integral_cast failed for " << source
       << " (would be " << static_cast<Dest>(source) << ")";

   return static_cast<Dest>(source);
 }

 // A version of dchecked_integral_cast casting between an integral type and an enum type.
 // When casting to an enum type, the cast does not check if the value corresponds to an enumerator.
 // When casting from an enum type, the target type can be omitted and the enum's underlying type
 // shall be used.

 template <typename Dest, typename Source>
 constexpr
 typename std::enable_if<!std::is_enum<Source>::value, Dest>::type
 enum_cast(Source value) {
   return static_cast<Dest>(
       dchecked_integral_cast<typename std::underlying_type<Dest>::type>(value));
 }

 template <typename Dest = void, typename Source>
 constexpr
 typename std::enable_if<std::is_enum<Source>::value,
                         typename std::conditional<std::is_same<Dest, void>::value,
                                                   std::underlying_type<Source>,
                                                   Identity<Dest>>::type>::type::type
 enum_cast(Source value) {
   using return_type = typename std::conditional<std::is_same<Dest, void>::value,
                                                 std::underlying_type<Source>,
                                                 Identity<Dest>>::type::type;
   return dchecked_integral_cast<return_type>(
       static_cast<typename std::underlying_type<Source>::type>(value));
 }

 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
 // that goes through uintptr_t to avoid treating the pointer as "signed."

 template <typename Dest, typename Source>
 inline Dest reinterpret_cast64(Source source) {
   // This is the overload for casting from int64_t/uint64_t to a pointer.
   static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value,
                 "Source must be int64_t or uint64_t.");
   static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
   // Check that we don't lose any non-0 bits here.
   DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
   return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
 }

 template <typename Dest, typename Source>
 inline Dest reinterpret_cast64(Source* ptr) {
   // This is the overload for casting from a pointer to int64_t/uint64_t.
   static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value,
                 "Dest must be int64_t or uint64_t.");
   static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
   return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
 }

 // A version of reinterpret_cast<>() between pointers and int32_t/uint32_t that enforces
 // zero-extension and checks that the values are converted without loss of precision.

 template <typename Dest, typename Source>
 inline Dest reinterpret_cast32(Source source) {
   // This is the overload for casting from int32_t/uint32_t to a pointer.
   static_assert(std::is_same<Source, int32_t>::value || std::is_same<Source, uint32_t>::value,
                 "Source must be int32_t or uint32_t.");
   static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
   // Check that we don't lose any non-0 bits here.
   static_assert(sizeof(uintptr_t) >= sizeof(Source), "Expecting at least 32-bit pointers.");
   return reinterpret_cast<Dest>(static_cast<uintptr_t>(static_cast<uint32_t>(source)));
 }

 template <typename Dest, typename Source>
 inline Dest reinterpret_cast32(Source* ptr) {
   // This is the overload for casting from a pointer to int32_t/uint32_t.
   static_assert(std::is_same<Dest, int32_t>::value || std::is_same<Dest, uint32_t>::value,
                 "Dest must be int32_t or uint32_t.");
   static_assert(sizeof(uintptr_t) >= sizeof(Dest), "Expecting at least 32-bit pointers.");
   return static_cast<Dest>(dchecked_integral_cast<uint32_t>(reinterpret_cast<uintptr_t>(ptr)));
 }

 }  // namespace art

 #endif  // ART_LIBARTBASE_BASE_CASTS_H_
	/*
	* Copyright (C) 2011 The Android Open Source Project
	*
	* 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
	*
	* http://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.
	*/

	#ifndef ART_LIBARTBASE_BASE_CASTS_H_
	#define ART_LIBARTBASE_BASE_CASTS_H_

	#include <assert.h>
	#include <stdint.h>
	#include <string.h>

	#include <limits>
	#include <type_traits>

	#include <android-base/logging.h>

	#include "stl_util_identity.h"

	namespace art {

	// Use implicit_cast as a safe version of static_cast or const_cast
	// for upcasting in the type hierarchy (i.e. casting a pointer to Foo
	// to a pointer to SuperclassOfFoo or casting a pointer to Foo to
	// a const pointer to Foo).
	// When you use implicit_cast, the compiler checks that the cast is safe.
	// Such explicit implicit_casts are necessary in surprisingly many
	// situations where C++ demands an exact type match instead of an
	// argument type convertible to a target type.
	//
	// The From type can be inferred, so the preferred syntax for using
	// implicit_cast is the same as for static_cast etc.:
	//
	// implicit_cast<ToType>(expr)
	//
	// implicit_cast would have been part of the C++ standard library,
	// but the proposal was submitted too late. It will probably make
	// its way into the language in the future.
	template<typename To, typename From>
	inline To implicit_cast(From const &f) {
	return f;
	}

	// When you upcast (that is, cast a pointer from type Foo to type
	// SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
	// always succeed. When you downcast (that is, cast a pointer from
	// type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
	// how do you know the pointer is really of type SubclassOfFoo? It
	// could be a bare Foo, or of type DifferentSubclassOfFoo. Thus,
	// when you downcast, you should use this macro. In debug mode, we
	// use dynamic_cast<> to double-check the downcast is legal (we die
	// if it's not). In normal mode, we do the efficient static_cast<>
	// instead. Thus, it's important to test in debug mode to make sure
	// the cast is legal!
	// This is the only place in the code we should use dynamic_cast<>.
	// In particular, you SHOULDN'T be using dynamic_cast<> in order to
	// do RTTI (eg code like this:
	// if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
	// if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
	// You should design the code some other way not to need this.

	template<typename To, typename From> // use like this: down_cast<T*>(foo);
	inline To down_cast(From* f) { // so we only accept pointers
	static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value,
	"down_cast unsafe as To is not a subtype of From");

	return static_cast<To>(f);
	}

	template<typename To, typename From> // use like this: down_cast<T&>(foo);
	inline To down_cast(From& f) { // so we only accept references
	static_assert(std::is_base_of<From, typename std::remove_reference<To>::type>::value,
	"down_cast unsafe as To is not a subtype of From");

	return static_cast<To>(f);
	}

	template <class Dest, class Source>
	inline Dest bit_cast(const Source& source) {
	// Compile time assertion: sizeof(Dest) == sizeof(Source)
	// A compile error here means your Dest and Source have different sizes.
	static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal");
	Dest dest;
	memcpy(&dest, &source, sizeof(dest));
	return dest;
	}

	// A version of static_cast that DCHECKs that the value can be precisely represented
	// when converting to Dest.
	template <typename Dest, typename Source>
	constexpr Dest dchecked_integral_cast(Source source) {
	DCHECK(
	// Check that the value is within the lower limit of Dest.
	(static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <=
	static_cast<intmax_t>(std::numeric_limits<Source>::min()) \|\|
	source >= static_cast<Source>(std::numeric_limits<Dest>::min())) &&
	// Check that the value is within the upper limit of Dest.
	(static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >=
	static_cast<uintmax_t>(std::numeric_limits<Source>::max()) \|\|
	source <= static_cast<Source>(std::numeric_limits<Dest>::max())))
	<< "dchecked_integral_cast failed for " << source
	<< " (would be " << static_cast<Dest>(source) << ")";

	return static_cast<Dest>(source);
	}

	// A version of dchecked_integral_cast casting between an integral type and an enum type.
	// When casting to an enum type, the cast does not check if the value corresponds to an enumerator.
	// When casting from an enum type, the target type can be omitted and the enum's underlying type
	// shall be used.

	template <typename Dest, typename Source>
	constexpr
	typename std::enable_if<!std::is_enum<Source>::value, Dest>::type
	enum_cast(Source value) {
	return static_cast<Dest>(
	dchecked_integral_cast<typename std::underlying_type<Dest>::type>(value));
	}

	template <typename Dest = void, typename Source>
	constexpr
	typename std::enable_if<std::is_enum<Source>::value,
	typename std::conditional<std::is_same<Dest, void>::value,
	std::underlying_type<Source>,
	Identity<Dest>>::type>::type::type
	enum_cast(Source value) {
	using return_type = typename std::conditional<std::is_same<Dest, void>::value,
	std::underlying_type<Source>,
	Identity<Dest>>::type::type;
	return dchecked_integral_cast<return_type>(
	static_cast<typename std::underlying_type<Source>::type>(value));
	}

	// A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
	// that goes through uintptr_t to avoid treating the pointer as "signed."

	template <typename Dest, typename Source>
	inline Dest reinterpret_cast64(Source source) {
	// This is the overload for casting from int64_t/uint64_t to a pointer.
	static_assert(std::is_same<Source, int64_t>::value \|\| std::is_same<Source, uint64_t>::value,
	"Source must be int64_t or uint64_t.");
	static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
	// Check that we don't lose any non-0 bits here.
	DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
	return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
	}

	template <typename Dest, typename Source>
	inline Dest reinterpret_cast64(Source* ptr) {
	// This is the overload for casting from a pointer to int64_t/uint64_t.
	static_assert(std::is_same<Dest, int64_t>::value \|\| std::is_same<Dest, uint64_t>::value,
	"Dest must be int64_t or uint64_t.");
	static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
	return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
	}

	// A version of reinterpret_cast<>() between pointers and int32_t/uint32_t that enforces
	// zero-extension and checks that the values are converted without loss of precision.

	template <typename Dest, typename Source>
	inline Dest reinterpret_cast32(Source source) {
	// This is the overload for casting from int32_t/uint32_t to a pointer.
	static_assert(std::is_same<Source, int32_t>::value \|\| std::is_same<Source, uint32_t>::value,
	"Source must be int32_t or uint32_t.");
	static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
	// Check that we don't lose any non-0 bits here.
	static_assert(sizeof(uintptr_t) >= sizeof(Source), "Expecting at least 32-bit pointers.");
	return reinterpret_cast<Dest>(static_cast<uintptr_t>(static_cast<uint32_t>(source)));
	}

	template <typename Dest, typename Source>
	inline Dest reinterpret_cast32(Source* ptr) {
	// This is the overload for casting from a pointer to int32_t/uint32_t.
	static_assert(std::is_same<Dest, int32_t>::value \|\| std::is_same<Dest, uint32_t>::value,
	"Dest must be int32_t or uint32_t.");
	static_assert(sizeof(uintptr_t) >= sizeof(Dest), "Expecting at least 32-bit pointers.");
	return static_cast<Dest>(dchecked_integral_cast<uint32_t>(reinterpret_cast<uintptr_t>(ptr)));
	}

	} // namespace art

	#endif // ART_LIBARTBASE_BASE_CASTS_H_