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 <assert.h>
21 #include <stdint.h>
22 #include <string.h>
23 
24 #include <limits>
25 #include <type_traits>
26 
27 #include <android-base/logging.h>
28 
29 #include "stl_util_identity.h"
30 
31 namespace art {
32 
33 // Use implicit_cast as a safe version of static_cast or const_cast
34 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo
35 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to
36 // a const pointer to Foo).
37 // When you use implicit_cast, the compiler checks that the cast is safe.
38 // Such explicit implicit_casts are necessary in surprisingly many
39 // situations where C++ demands an exact type match instead of an
40 // argument type convertible to a target type.
41 //
42 // The From type can be inferred, so the preferred syntax for using
43 // implicit_cast is the same as for static_cast etc.:
44 //
45 //   implicit_cast<ToType>(expr)
46 //
47 // implicit_cast would have been part of the C++ standard library,
48 // but the proposal was submitted too late.  It will probably make
49 // its way into the language in the future.
50 template<typename To, typename From>
implicit_cast(From const & f)51 inline To implicit_cast(From const &f) {
52   return f;
53 }
54 
55 // When you upcast (that is, cast a pointer from type Foo to type
56 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
57 // always succeed.  When you downcast (that is, cast a pointer from
58 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
59 // how do you know the pointer is really of type SubclassOfFoo?  It
60 // could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
61 // when you downcast, you should use this macro.  In debug mode, we
62 // use dynamic_cast<> to double-check the downcast is legal (we die
63 // if it's not).  In normal mode, we do the efficient static_cast<>
64 // instead.  Thus, it's important to test in debug mode to make sure
65 // the cast is legal!
66 //    This is the only place in the code we should use dynamic_cast<>.
67 // In particular, you SHOULDN'T be using dynamic_cast<> in order to
68 // do RTTI (eg code like this:
69 //    if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
70 //    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
71 // You should design the code some other way not to need this.
72 
73 template<typename To, typename From>     // use like this: down_cast<T*>(foo);
down_cast(From * f)74 inline To down_cast(From* f) {                   // so we only accept pointers
75   static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value,
76                 "down_cast unsafe as To is not a subtype of From");
77 
78   return static_cast<To>(f);
79 }
80 
81 template<typename To, typename From>     // use like this: down_cast<T&>(foo);
down_cast(From & f)82 inline To down_cast(From& f) {           // so we only accept references
83   static_assert(std::is_base_of<From, typename std::remove_reference<To>::type>::value,
84                 "down_cast unsafe as To is not a subtype of From");
85 
86   return static_cast<To>(f);
87 }
88 
89 template <class Dest, class Source>
bit_cast(const Source & source)90 inline Dest bit_cast(const Source& source) {
91   // Compile time assertion: sizeof(Dest) == sizeof(Source)
92   // A compile error here means your Dest and Source have different sizes.
93   static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal");
94   Dest dest;
95   memcpy(&dest, &source, sizeof(dest));
96   return dest;
97 }
98 
99 // A version of static_cast that DCHECKs that the value can be precisely represented
100 // when converting to Dest.
101 template <typename Dest, typename Source>
dchecked_integral_cast(Source source)102 constexpr Dest dchecked_integral_cast(Source source) {
103   DCHECK(
104       // Check that the value is within the lower limit of Dest.
105       (static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <=
106           static_cast<intmax_t>(std::numeric_limits<Source>::min()) ||
107           source >= static_cast<Source>(std::numeric_limits<Dest>::min())) &&
108       // Check that the value is within the upper limit of Dest.
109       (static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >=
110           static_cast<uintmax_t>(std::numeric_limits<Source>::max()) ||
111           source <= static_cast<Source>(std::numeric_limits<Dest>::max())))
112       << "dchecked_integral_cast failed for " << source
113       << " (would be " << static_cast<Dest>(source) << ")";
114 
115   return static_cast<Dest>(source);
116 }
117 
118 // A version of dchecked_integral_cast casting between an integral type and an enum type.
119 // When casting to an enum type, the cast does not check if the value corresponds to an enumerator.
120 // When casting from an enum type, the target type can be omitted and the enum's underlying type
121 // shall be used.
122 
123 template <typename Dest, typename Source>
124 constexpr
125 typename std::enable_if<!std::is_enum<Source>::value, Dest>::type
enum_cast(Source value)126 enum_cast(Source value) {
127   return static_cast<Dest>(
128       dchecked_integral_cast<typename std::underlying_type<Dest>::type>(value));
129 }
130 
131 template <typename Dest = void, typename Source>
132 constexpr
133 typename std::enable_if<std::is_enum<Source>::value,
134                         typename std::conditional<std::is_same<Dest, void>::value,
135                                                   std::underlying_type<Source>,
136                                                   Identity<Dest>>::type>::type::type
enum_cast(Source value)137 enum_cast(Source value) {
138   using return_type = typename std::conditional<std::is_same<Dest, void>::value,
139                                                 std::underlying_type<Source>,
140                                                 Identity<Dest>>::type::type;
141   return dchecked_integral_cast<return_type>(
142       static_cast<typename std::underlying_type<Source>::type>(value));
143 }
144 
145 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
146 // that goes through uintptr_t to avoid treating the pointer as "signed."
147 
148 template <typename Dest, typename Source>
reinterpret_cast64(Source source)149 inline Dest reinterpret_cast64(Source source) {
150   // This is the overload for casting from int64_t/uint64_t to a pointer.
151   static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value,
152                 "Source must be int64_t or uint64_t.");
153   static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
154   // Check that we don't lose any non-0 bits here.
155   DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
156   return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
157 }
158 
159 template <typename Dest, typename Source>
reinterpret_cast64(Source * ptr)160 inline Dest reinterpret_cast64(Source* ptr) {
161   // This is the overload for casting from a pointer to int64_t/uint64_t.
162   static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value,
163                 "Dest must be int64_t or uint64_t.");
164   static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
165   return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
166 }
167 
168 // A version of reinterpret_cast<>() between pointers and int32_t/uint32_t that enforces
169 // zero-extension and checks that the values are converted without loss of precision.
170 
171 template <typename Dest, typename Source>
reinterpret_cast32(Source source)172 inline Dest reinterpret_cast32(Source source) {
173   // This is the overload for casting from int32_t/uint32_t to a pointer.
174   static_assert(std::is_same<Source, int32_t>::value || std::is_same<Source, uint32_t>::value,
175                 "Source must be int32_t or uint32_t.");
176   static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
177   // Check that we don't lose any non-0 bits here.
178   static_assert(sizeof(uintptr_t) >= sizeof(Source), "Expecting at least 32-bit pointers.");
179   return reinterpret_cast<Dest>(static_cast<uintptr_t>(static_cast<uint32_t>(source)));
180 }
181 
182 template <typename Dest, typename Source>
reinterpret_cast32(Source * ptr)183 inline Dest reinterpret_cast32(Source* ptr) {
184   // This is the overload for casting from a pointer to int32_t/uint32_t.
185   static_assert(std::is_same<Dest, int32_t>::value || std::is_same<Dest, uint32_t>::value,
186                 "Dest must be int32_t or uint32_t.");
187   static_assert(sizeof(uintptr_t) >= sizeof(Dest), "Expecting at least 32-bit pointers.");
188   return static_cast<Dest>(dchecked_integral_cast<uint32_t>(reinterpret_cast<uintptr_t>(ptr)));
189 }
190 
191 }  // namespace art
192 
193 #endif  // ART_LIBARTBASE_BASE_CASTS_H_
194