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 <string.h>
22#include "base/macros.h"
23
24namespace art {
25
26// Use implicit_cast as a safe version of static_cast or const_cast
27// for upcasting in the type hierarchy (i.e. casting a pointer to Foo
28// to a pointer to SuperclassOfFoo or casting a pointer to Foo to
29// a const pointer to Foo).
30// When you use implicit_cast, the compiler checks that the cast is safe.
31// Such explicit implicit_casts are necessary in surprisingly many
32// situations where C++ demands an exact type match instead of an
33// argument type convertable to a target type.
34//
35// The From type can be inferred, so the preferred syntax for using
36// implicit_cast is the same as for static_cast etc.:
37//
38//   implicit_cast<ToType>(expr)
39//
40// implicit_cast would have been part of the C++ standard library,
41// but the proposal was submitted too late.  It will probably make
42// its way into the language in the future.
43template<typename To, typename From>
44inline To implicit_cast(From const &f) {
45  return f;
46}
47
48// When you upcast (that is, cast a pointer from type Foo to type
49// SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
50// always succeed.  When you downcast (that is, cast a pointer from
51// type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
52// how do you know the pointer is really of type SubclassOfFoo?  It
53// could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
54// when you downcast, you should use this macro.  In debug mode, we
55// use dynamic_cast<> to double-check the downcast is legal (we die
56// if it's not).  In normal mode, we do the efficient static_cast<>
57// instead.  Thus, it's important to test in debug mode to make sure
58// the cast is legal!
59//    This is the only place in the code we should use dynamic_cast<>.
60// In particular, you SHOULDN'T be using dynamic_cast<> in order to
61// do RTTI (eg code like this:
62//    if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
63//    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
64// You should design the code some other way not to need this.
65
66template<typename To, typename From>     // use like this: down_cast<T*>(foo);
67inline To down_cast(From* f) {                   // so we only accept pointers
68  // Ensures that To is a sub-type of From *.  This test is here only
69  // for compile-time type checking, and has no overhead in an
70  // optimized build at run-time, as it will be optimized away
71  // completely.
72  if (false) {
73    implicit_cast<From*, To>(0);
74  }
75
76  //
77  // assert(f == NULL || dynamic_cast<To>(f) != NULL);  // RTTI: debug mode only!
78  return static_cast<To>(f);
79}
80
81template <class Dest, class Source>
82inline Dest bit_cast(const Source& source) {
83  // Compile time assertion: sizeof(Dest) == sizeof(Source)
84  // A compile error here means your Dest and Source have different sizes.
85  COMPILE_ASSERT(sizeof(Dest) == sizeof(Source), verify_sizes_are_equal);
86  Dest dest;
87  memcpy(&dest, &source, sizeof(dest));
88  return dest;
89}
90
91}  // namespace art
92
93#endif  // ART_RUNTIME_BASE_CASTS_H_
94