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 <string.h> 22 #include "base/macros.h" 23 24 namespace art { 25 26 // Use implicit_cast as a safe version of static_cast or const_cast 27 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo 28 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to 29 // a const pointer to Foo). 30 // When you use implicit_cast, the compiler checks that the cast is safe. 31 // Such explicit implicit_casts are necessary in surprisingly many 32 // situations where C++ demands an exact type match instead of an 33 // argument type convertable to a target type. 34 // 35 // The From type can be inferred, so the preferred syntax for using 36 // implicit_cast is the same as for static_cast etc.: 37 // 38 // implicit_cast<ToType>(expr) 39 // 40 // implicit_cast would have been part of the C++ standard library, 41 // but the proposal was submitted too late. It will probably make 42 // its way into the language in the future. 43 template<typename To, typename From> 44 inline To implicit_cast(From const &f) { 45 return f; 46 } 47 48 // When you upcast (that is, cast a pointer from type Foo to type 49 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts 50 // always succeed. When you downcast (that is, cast a pointer from 51 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because 52 // how do you know the pointer is really of type SubclassOfFoo? It 53 // could be a bare Foo, or of type DifferentSubclassOfFoo. Thus, 54 // when you downcast, you should use this macro. In debug mode, we 55 // use dynamic_cast<> to double-check the downcast is legal (we die 56 // if it's not). In normal mode, we do the efficient static_cast<> 57 // instead. Thus, it's important to test in debug mode to make sure 58 // the cast is legal! 59 // This is the only place in the code we should use dynamic_cast<>. 60 // In particular, you SHOULDN'T be using dynamic_cast<> in order to 61 // do RTTI (eg code like this: 62 // if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo); 63 // if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo); 64 // You should design the code some other way not to need this. 65 66 template<typename To, typename From> // use like this: down_cast<T*>(foo); 67 inline To down_cast(From* f) { // so we only accept pointers 68 // Ensures that To is a sub-type of From *. This test is here only 69 // for compile-time type checking, and has no overhead in an 70 // optimized build at run-time, as it will be optimized away 71 // completely. 72 if (false) { 73 implicit_cast<From*, To>(0); 74 } 75 76 // 77 // assert(f == NULL || dynamic_cast<To>(f) != NULL); // RTTI: debug mode only! 78 return static_cast<To>(f); 79 } 80 81 template <class Dest, class Source> 82 inline Dest bit_cast(const Source& source) { 83 // Compile time assertion: sizeof(Dest) == sizeof(Source) 84 // A compile error here means your Dest and Source have different sizes. 85 COMPILE_ASSERT(sizeof(Dest) == sizeof(Source), verify_sizes_are_equal); 86 Dest dest; 87 memcpy(&dest, &source, sizeof(dest)); 88 return dest; 89 } 90 91 } // namespace art 92 93 #endif // ART_RUNTIME_BASE_CASTS_H_ 94
