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_v<From, std::remove_pointer_t<To>>,
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_v<From, std::remove_reference_t<To>>,
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
enum_cast(Source value)124 std::enable_if_t<!std::is_enum_v<Source>, Dest> enum_cast(Source value) {
125 return static_cast<Dest>(dchecked_integral_cast<std::underlying_type_t<Dest>>(value));
126 }
127
128 template <typename Dest = void, typename Source>
129 constexpr
130 typename std::enable_if_t<std::is_enum_v<Source>,
131 std::conditional_t<std::is_same_v<Dest, void>,
132 std::underlying_type<Source>,
133 Identity<Dest>>>::type
enum_cast(Source value)134 enum_cast(Source value) {
135 using return_type = typename std::conditional_t<std::is_same_v<Dest, void>,
136 std::underlying_type<Source>,
137 Identity<Dest>>::type;
138 return dchecked_integral_cast<return_type>(
139 static_cast<std::underlying_type_t<Source>>(value));
140 }
141
142 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
143 // that goes through uintptr_t to avoid treating the pointer as "signed."
144
145 template <typename Dest, typename Source>
reinterpret_cast64(Source source)146 inline Dest reinterpret_cast64(Source source) {
147 // This is the overload for casting from int64_t/uint64_t to a pointer.
148 static_assert(std::is_same_v<Source, int64_t> || std::is_same_v<Source, uint64_t>,
149 "Source must be int64_t or uint64_t.");
150 static_assert(std::is_pointer_v<Dest>, "Dest must be a pointer.");
151 // Check that we don't lose any non-0 bits here.
152 DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
153 return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
154 }
155
156 template <typename Dest, typename Source>
reinterpret_cast64(Source * ptr)157 inline Dest reinterpret_cast64(Source* ptr) {
158 // This is the overload for casting from a pointer to int64_t/uint64_t.
159 static_assert(std::is_same_v<Dest, int64_t> || std::is_same_v<Dest, uint64_t>,
160 "Dest must be int64_t or uint64_t.");
161 static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
162 return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
163 }
164
165 // A version of reinterpret_cast<>() between pointers and int32_t/uint32_t that enforces
166 // zero-extension and checks that the values are converted without loss of precision.
167
168 template <typename Dest, typename Source>
reinterpret_cast32(Source source)169 inline Dest reinterpret_cast32(Source source) {
170 // This is the overload for casting from int32_t/uint32_t to a pointer.
171 static_assert(std::is_same_v<Source, int32_t> || std::is_same_v<Source, uint32_t>,
172 "Source must be int32_t or uint32_t.");
173 static_assert(std::is_pointer_v<Dest>, "Dest must be a pointer.");
174 // Check that we don't lose any non-0 bits here.
175 static_assert(sizeof(uintptr_t) >= sizeof(Source), "Expecting at least 32-bit pointers.");
176 return reinterpret_cast<Dest>(static_cast<uintptr_t>(static_cast<uint32_t>(source)));
177 }
178
179 template <typename Dest, typename Source>
reinterpret_cast32(Source * ptr)180 inline Dest reinterpret_cast32(Source* ptr) {
181 // This is the overload for casting from a pointer to int32_t/uint32_t.
182 static_assert(std::is_same_v<Dest, int32_t> || std::is_same_v<Dest, uint32_t>,
183 "Dest must be int32_t or uint32_t.");
184 static_assert(sizeof(uintptr_t) >= sizeof(Dest), "Expecting at least 32-bit pointers.");
185 return static_cast<Dest>(dchecked_integral_cast<uint32_t>(reinterpret_cast<uintptr_t>(ptr)));
186 }
187
188 } // namespace art
189
190 #endif // ART_LIBARTBASE_BASE_CASTS_H_
191