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