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
31namespace 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.
50template<typename To, typename From>
51inline 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
73template<typename To, typename From>     // use like this: down_cast<T*>(foo);
74inline 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
81template<typename To, typename From>     // use like this: down_cast<T&>(foo);
82inline 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
89template <class Dest, class Source>
90inline 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.
101template <typename Dest, typename Source>
102constexpr 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
123template <typename Dest, typename Source>
124constexpr
125typename std::enable_if<!std::is_enum<Source>::value, Dest>::type
126enum_cast(Source value) {
127  return static_cast<Dest>(
128      dchecked_integral_cast<typename std::underlying_type<Dest>::type>(value));
129}
130
131template <typename Dest = void, typename Source>
132constexpr
133typename 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
137enum_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
148template <typename Dest, typename Source>
149inline 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
159template <typename Dest, typename Source>
160inline 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}  // namespace art
169
170#endif  // ART_LIBARTBASE_BASE_CASTS_H_
171