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