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_RUNTIME_BASE_CASTS_H_ 18 #define ART_RUNTIME_BASE_CASTS_H_ 19 20 #include <assert.h> 21 #include <limits> 22 #include <stdint.h> 23 #include <string.h> 24 #include <type_traits> 25 26 #include "base/logging.h" 27 #include "base/macros.h" 28 29 namespace art { 30 31 // Use implicit_cast as a safe version of static_cast or const_cast 32 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo 33 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to 34 // a const pointer to Foo). 35 // When you use implicit_cast, the compiler checks that the cast is safe. 36 // Such explicit implicit_casts are necessary in surprisingly many 37 // situations where C++ demands an exact type match instead of an 38 // argument type convertible to a target type. 39 // 40 // The From type can be inferred, so the preferred syntax for using 41 // implicit_cast is the same as for static_cast etc.: 42 // 43 // implicit_cast<ToType>(expr) 44 // 45 // implicit_cast would have been part of the C++ standard library, 46 // but the proposal was submitted too late. It will probably make 47 // its way into the language in the future. 48 template<typename To, typename From> 49 inline To implicit_cast(From const &f) { 50 return f; 51 } 52 53 // When you upcast (that is, cast a pointer from type Foo to type 54 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts 55 // always succeed. When you downcast (that is, cast a pointer from 56 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because 57 // how do you know the pointer is really of type SubclassOfFoo? It 58 // could be a bare Foo, or of type DifferentSubclassOfFoo. Thus, 59 // when you downcast, you should use this macro. In debug mode, we 60 // use dynamic_cast<> to double-check the downcast is legal (we die 61 // if it's not). In normal mode, we do the efficient static_cast<> 62 // instead. Thus, it's important to test in debug mode to make sure 63 // the cast is legal! 64 // This is the only place in the code we should use dynamic_cast<>. 65 // In particular, you SHOULDN'T be using dynamic_cast<> in order to 66 // do RTTI (eg code like this: 67 // if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo); 68 // if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo); 69 // You should design the code some other way not to need this. 70 71 template<typename To, typename From> // use like this: down_cast<T*>(foo); 72 inline To down_cast(From* f) { // so we only accept pointers 73 static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value, 74 "down_cast unsafe as To is not a subtype of From"); 75 76 return static_cast<To>(f); 77 } 78 79 template <class Dest, class Source> 80 inline Dest bit_cast(const Source& source) { 81 // Compile time assertion: sizeof(Dest) == sizeof(Source) 82 // A compile error here means your Dest and Source have different sizes. 83 static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal"); 84 Dest dest; 85 memcpy(&dest, &source, sizeof(dest)); 86 return dest; 87 } 88 89 // A version of static_cast that DCHECKs that the value can be precisely represented 90 // when converting to Dest. 91 template <typename Dest, typename Source> 92 inline Dest dchecked_integral_cast(const Source source) { 93 DCHECK( 94 // Check that the value is within the lower limit of Dest. 95 (static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <= 96 static_cast<intmax_t>(std::numeric_limits<Source>::min()) || 97 source >= static_cast<Source>(std::numeric_limits<Dest>::min())) && 98 // Check that the value is within the upper limit of Dest. 99 (static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >= 100 static_cast<uintmax_t>(std::numeric_limits<Source>::max()) || 101 source <= static_cast<Source>(std::numeric_limits<Dest>::max()))); 102 103 return static_cast<Dest>(source); 104 } 105 106 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t 107 // that goes through uintptr_t to avoid treating the pointer as "signed." 108 109 template <typename Dest, typename Source> 110 inline Dest reinterpret_cast64(Source source) { 111 // This is the overload for casting from int64_t/uint64_t to a pointer. 112 static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value, 113 "Source must be int64_t or uint64_t."); 114 static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer."); 115 // Check that we don't lose any non-0 bits here. 116 DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source); 117 return reinterpret_cast<Dest>(static_cast<uintptr_t>(source)); 118 } 119 120 template <typename Dest, typename Source> 121 inline Dest reinterpret_cast64(Source* ptr) { 122 // This is the overload for casting from a pointer to int64_t/uint64_t. 123 static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value, 124 "Dest must be int64_t or uint64_t."); 125 static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers."); 126 return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr)); 127 } 128 129 } // namespace art 130 131 #endif // ART_RUNTIME_BASE_CASTS_H_ 132
