1 /* 2 * Copyright (C) 2011 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef ART_LIBARTBASE_BASE_CASTS_H_ 18 #define ART_LIBARTBASE_BASE_CASTS_H_ 19 20 #include <stdint.h> 21 #include <string.h> 22 23 #include <limits> 24 #include <type_traits> 25 26 #include <android-base/logging.h> 27 28 #include "stl_util_identity.h" 29 30 namespace art { 31 32 // Use implicit_cast as a safe version of static_cast or const_cast 33 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo 34 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to 35 // a const pointer to Foo). 36 // When you use implicit_cast, the compiler checks that the cast is safe. 37 // Such explicit implicit_casts are necessary in surprisingly many 38 // situations where C++ demands an exact type match instead of an 39 // argument type convertible to a target type. 40 // 41 // The From type can be inferred, so the preferred syntax for using 42 // implicit_cast is the same as for static_cast etc.: 43 // 44 // implicit_cast<ToType>(expr) 45 // 46 // implicit_cast would have been part of the C++ standard library, 47 // but the proposal was submitted too late. It will probably make 48 // its way into the language in the future. 49 template<typename To, typename From> 50 inline To implicit_cast(From const &f) { 51 return f; 52 } 53 54 // When you upcast (that is, cast a pointer from type Foo to type 55 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts 56 // always succeed. When you downcast (that is, cast a pointer from 57 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because 58 // how do you know the pointer is really of type SubclassOfFoo? It 59 // could be a bare Foo, or of type DifferentSubclassOfFoo. Thus, 60 // when you downcast, you should use this macro. In debug mode, we 61 // use dynamic_cast<> to double-check the downcast is legal (we die 62 // if it's not). In normal mode, we do the efficient static_cast<> 63 // instead. Thus, it's important to test in debug mode to make sure 64 // the cast is legal! 65 // This is the only place in the code we should use dynamic_cast<>. 66 // In particular, you SHOULDN'T be using dynamic_cast<> in order to 67 // do RTTI (eg code like this: 68 // if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo); 69 // if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo); 70 // You should design the code some other way not to need this. 71 72 template<typename To, typename From> // use like this: down_cast<T*>(foo); 73 inline To down_cast(From* f) { // so we only accept pointers 74 static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value, 75 "down_cast unsafe as To is not a subtype of From"); 76 77 return static_cast<To>(f); 78 } 79 80 template<typename To, typename From> // use like this: down_cast<T&>(foo); 81 inline To down_cast(From& f) { // so we only accept references 82 static_assert(std::is_base_of<From, typename std::remove_reference<To>::type>::value, 83 "down_cast unsafe as To is not a subtype of From"); 84 85 return static_cast<To>(f); 86 } 87 88 template <class Dest, class Source> 89 inline Dest bit_cast(const Source& source) { 90 // Compile time assertion: sizeof(Dest) == sizeof(Source) 91 // A compile error here means your Dest and Source have different sizes. 92 static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal"); 93 Dest dest; 94 memcpy(&dest, &source, sizeof(dest)); 95 return dest; 96 } 97 98 // A version of static_cast that DCHECKs that the value can be precisely represented 99 // when converting to Dest. 100 template <typename Dest, typename Source> 101 constexpr Dest dchecked_integral_cast(Source source) { 102 DCHECK( 103 // Check that the value is within the lower limit of Dest. 104 (static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <= 105 static_cast<intmax_t>(std::numeric_limits<Source>::min()) || 106 source >= static_cast<Source>(std::numeric_limits<Dest>::min())) && 107 // Check that the value is within the upper limit of Dest. 108 (static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >= 109 static_cast<uintmax_t>(std::numeric_limits<Source>::max()) || 110 source <= static_cast<Source>(std::numeric_limits<Dest>::max()))) 111 << "dchecked_integral_cast failed for " << source 112 << " (would be " << static_cast<Dest>(source) << ")"; 113 114 return static_cast<Dest>(source); 115 } 116 117 // A version of dchecked_integral_cast casting between an integral type and an enum type. 118 // When casting to an enum type, the cast does not check if the value corresponds to an enumerator. 119 // When casting from an enum type, the target type can be omitted and the enum's underlying type 120 // shall be used. 121 122 template <typename Dest, typename Source> 123 constexpr 124 typename std::enable_if<!std::is_enum<Source>::value, Dest>::type 125 enum_cast(Source value) { 126 return static_cast<Dest>( 127 dchecked_integral_cast<typename std::underlying_type<Dest>::type>(value)); 128 } 129 130 template <typename Dest = void, typename Source> 131 constexpr 132 typename std::enable_if<std::is_enum<Source>::value, 133 typename std::conditional<std::is_same<Dest, void>::value, 134 std::underlying_type<Source>, 135 Identity<Dest>>::type>::type::type 136 enum_cast(Source value) { 137 using return_type = typename std::conditional<std::is_same<Dest, void>::value, 138 std::underlying_type<Source>, 139 Identity<Dest>>::type::type; 140 return dchecked_integral_cast<return_type>( 141 static_cast<typename std::underlying_type<Source>::type>(value)); 142 } 143 144 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t 145 // that goes through uintptr_t to avoid treating the pointer as "signed." 146 147 template <typename Dest, typename Source> 148 inline Dest reinterpret_cast64(Source source) { 149 // This is the overload for casting from int64_t/uint64_t to a pointer. 150 static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value, 151 "Source must be int64_t or uint64_t."); 152 static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer."); 153 // Check that we don't lose any non-0 bits here. 154 DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source); 155 return reinterpret_cast<Dest>(static_cast<uintptr_t>(source)); 156 } 157 158 template <typename Dest, typename Source> 159 inline Dest reinterpret_cast64(Source* ptr) { 160 // This is the overload for casting from a pointer to int64_t/uint64_t. 161 static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value, 162 "Dest must be int64_t or uint64_t."); 163 static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers."); 164 return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr)); 165 } 166 167 // A version of reinterpret_cast<>() between pointers and int32_t/uint32_t that enforces 168 // zero-extension and checks that the values are converted without loss of precision. 169 170 template <typename Dest, typename Source> 171 inline Dest reinterpret_cast32(Source source) { 172 // This is the overload for casting from int32_t/uint32_t to a pointer. 173 static_assert(std::is_same<Source, int32_t>::value || std::is_same<Source, uint32_t>::value, 174 "Source must be int32_t or uint32_t."); 175 static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer."); 176 // Check that we don't lose any non-0 bits here. 177 static_assert(sizeof(uintptr_t) >= sizeof(Source), "Expecting at least 32-bit pointers."); 178 return reinterpret_cast<Dest>(static_cast<uintptr_t>(static_cast<uint32_t>(source))); 179 } 180 181 template <typename Dest, typename Source> 182 inline Dest reinterpret_cast32(Source* ptr) { 183 // This is the overload for casting from a pointer to int32_t/uint32_t. 184 static_assert(std::is_same<Dest, int32_t>::value || std::is_same<Dest, uint32_t>::value, 185 "Dest must be int32_t or uint32_t."); 186 static_assert(sizeof(uintptr_t) >= sizeof(Dest), "Expecting at least 32-bit pointers."); 187 return static_cast<Dest>(dchecked_integral_cast<uint32_t>(reinterpret_cast<uintptr_t>(ptr))); 188 } 189 190 } // namespace art 191 192 #endif // ART_LIBARTBASE_BASE_CASTS_H_ 193