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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>
implicit_cast(From const & f)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);
down_cast(From * f)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);
down_cast(From & f)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>
bit_cast(const Source & 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>
dchecked_integral_cast(Source 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
enum_cast(Source value)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
enum_cast(Source value)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>
reinterpret_cast64(Source 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>
reinterpret_cast64(Source * ptr)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>
reinterpret_cast32(Source 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>
reinterpret_cast32(Source * ptr)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