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 <stdint.h>
23 #include <string.h>
24 #include <type_traits>
25 
26 #include "base/logging.h"
27 #include "base/macros.h"
28 
29 namespace art {
30 
31 // Use implicit_cast as a safe version of static_cast or const_cast
32 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo
33 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to
34 // a const pointer to Foo).
35 // When you use implicit_cast, the compiler checks that the cast is safe.
36 // Such explicit implicit_casts are necessary in surprisingly many
37 // situations where C++ demands an exact type match instead of an
38 // argument type convertible to a target type.
39 //
40 // The From type can be inferred, so the preferred syntax for using
41 // implicit_cast is the same as for static_cast etc.:
42 //
43 //   implicit_cast<ToType>(expr)
44 //
45 // implicit_cast would have been part of the C++ standard library,
46 // but the proposal was submitted too late.  It will probably make
47 // its way into the language in the future.
48 template<typename To, typename From>
implicit_cast(From const & f)49 inline To implicit_cast(From const &f) {
50   return f;
51 }
52 
53 // When you upcast (that is, cast a pointer from type Foo to type
54 // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
55 // always succeed.  When you downcast (that is, cast a pointer from
56 // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
57 // how do you know the pointer is really of type SubclassOfFoo?  It
58 // could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
59 // when you downcast, you should use this macro.  In debug mode, we
60 // use dynamic_cast<> to double-check the downcast is legal (we die
61 // if it's not).  In normal mode, we do the efficient static_cast<>
62 // instead.  Thus, it's important to test in debug mode to make sure
63 // the cast is legal!
64 //    This is the only place in the code we should use dynamic_cast<>.
65 // In particular, you SHOULDN'T be using dynamic_cast<> in order to
66 // do RTTI (eg code like this:
67 //    if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
68 //    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
69 // You should design the code some other way not to need this.
70 
71 template<typename To, typename From>     // use like this: down_cast<T*>(foo);
down_cast(From * f)72 inline To down_cast(From* f) {                   // so we only accept pointers
73   static_assert(std::is_base_of<From, typename std::remove_pointer<To>::type>::value,
74                 "down_cast unsafe as To is not a subtype of From");
75 
76   return static_cast<To>(f);
77 }
78 
79 template <class Dest, class Source>
bit_cast(const Source & source)80 inline Dest bit_cast(const Source& source) {
81   // Compile time assertion: sizeof(Dest) == sizeof(Source)
82   // A compile error here means your Dest and Source have different sizes.
83   static_assert(sizeof(Dest) == sizeof(Source), "sizes should be equal");
84   Dest dest;
85   memcpy(&dest, &source, sizeof(dest));
86   return dest;
87 }
88 
89 // A version of static_cast that DCHECKs that the value can be precisely represented
90 // when converting to Dest.
91 template <typename Dest, typename Source>
dchecked_integral_cast(const Source source)92 inline Dest dchecked_integral_cast(const Source source) {
93   DCHECK(
94       // Check that the value is within the lower limit of Dest.
95       (static_cast<intmax_t>(std::numeric_limits<Dest>::min()) <=
96           static_cast<intmax_t>(std::numeric_limits<Source>::min()) ||
97           source >= static_cast<Source>(std::numeric_limits<Dest>::min())) &&
98       // Check that the value is within the upper limit of Dest.
99       (static_cast<uintmax_t>(std::numeric_limits<Dest>::max()) >=
100           static_cast<uintmax_t>(std::numeric_limits<Source>::max()) ||
101           source <= static_cast<Source>(std::numeric_limits<Dest>::max())));
102 
103   return static_cast<Dest>(source);
104 }
105 
106 // A version of reinterpret_cast<>() between pointers and int64_t/uint64_t
107 // that goes through uintptr_t to avoid treating the pointer as "signed."
108 
109 template <typename Dest, typename Source>
reinterpret_cast64(Source source)110 inline Dest reinterpret_cast64(Source source) {
111   // This is the overload for casting from int64_t/uint64_t to a pointer.
112   static_assert(std::is_same<Source, int64_t>::value || std::is_same<Source, uint64_t>::value,
113                 "Source must be int64_t or uint64_t.");
114   static_assert(std::is_pointer<Dest>::value, "Dest must be a pointer.");
115   // Check that we don't lose any non-0 bits here.
116   DCHECK_EQ(static_cast<Source>(static_cast<uintptr_t>(source)), source);
117   return reinterpret_cast<Dest>(static_cast<uintptr_t>(source));
118 }
119 
120 template <typename Dest, typename Source>
reinterpret_cast64(Source * ptr)121 inline Dest reinterpret_cast64(Source* ptr) {
122   // This is the overload for casting from a pointer to int64_t/uint64_t.
123   static_assert(std::is_same<Dest, int64_t>::value || std::is_same<Dest, uint64_t>::value,
124                 "Dest must be int64_t or uint64_t.");
125   static_assert(sizeof(uintptr_t) <= sizeof(Dest), "Expecting at most 64-bit pointers.");
126   return static_cast<Dest>(reinterpret_cast<uintptr_t>(ptr));
127 }
128 
129 }  // namespace art
130 
131 #endif  // ART_RUNTIME_BASE_CASTS_H_
132