1// Protocol Buffers - Google's data interchange format
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30
31#ifndef GOOGLE_PROTOBUF_CASTS_H__
32#define GOOGLE_PROTOBUF_CASTS_H__
33
34#include <google/protobuf/stubs/common.h>
35
36#include <google/protobuf/port_def.inc>
37#include <type_traits>
38
39namespace google {
40namespace protobuf {
41namespace internal {
42
43// Use implicit_cast as a safe version of static_cast or const_cast
44// for upcasting in the type hierarchy (i.e. casting a pointer to Foo
45// to a pointer to SuperclassOfFoo or casting a pointer to Foo to
46// a const pointer to Foo).
47// When you use implicit_cast, the compiler checks that the cast is safe.
48// Such explicit implicit_casts are necessary in surprisingly many
49// situations where C++ demands an exact type match instead of an
50// argument type convertable to a target type.
51//
52// The From type can be inferred, so the preferred syntax for using
53// implicit_cast is the same as for static_cast etc.:
54//
55// implicit_cast<ToType>(expr)
56//
57// implicit_cast would have been part of the C++ standard library,
58// but the proposal was submitted too late. It will probably make
59// its way into the language in the future.
60template<typename To, typename From>
61inline To implicit_cast(From const &f) {
62 return f;
63}
64
65// When you upcast (that is, cast a pointer from type Foo to type
66// SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
67// always succeed. When you downcast (that is, cast a pointer from
68// type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
69// how do you know the pointer is really of type SubclassOfFoo? It
70// could be a bare Foo, or of type DifferentSubclassOfFoo. Thus,
71// when you downcast, you should use this macro. In debug mode, we
72// use dynamic_cast<> to double-check the downcast is legal (we die
73// if it's not). In normal mode, we do the efficient static_cast<>
74// instead. Thus, it's important to test in debug mode to make sure
75// the cast is legal!
76// This is the only place in the code we should use dynamic_cast<>.
77// In particular, you SHOULDN'T be using dynamic_cast<> in order to
78// do RTTI (eg code like this:
79// if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
80// if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
81// You should design the code some other way not to need this.
82
83template<typename To, typename From> // use like this: down_cast<T*>(foo);
84inline To down_cast(From* f) { // so we only accept pointers
85 // Ensures that To is a sub-type of From *. This test is here only
86 // for compile-time type checking, and has no overhead in an
87 // optimized build at run-time, as it will be optimized away
88 // completely.
89 if (false) {
90 implicit_cast<From*, To>(0);
91 }
92
93#if !defined(NDEBUG) && PROTOBUF_RTTI
94 assert(f == nullptr || dynamic_cast<To>(f) != nullptr); // RTTI: debug mode only!
95#endif
96 return static_cast<To>(f);
97}
98
99template<typename To, typename From> // use like this: down_cast<T&>(foo);
100inline To down_cast(From& f) {
101 typedef typename std::remove_reference<To>::type* ToAsPointer;
102 // Ensures that To is a sub-type of From *. This test is here only
103 // for compile-time type checking, and has no overhead in an
104 // optimized build at run-time, as it will be optimized away
105 // completely.
106 if (false) {
107 implicit_cast<From*, ToAsPointer>(0);
108 }
109
110#if !defined(NDEBUG) && PROTOBUF_RTTI
111 // RTTI: debug mode only!
112 assert(dynamic_cast<ToAsPointer>(&f) != nullptr);
113#endif
114 return *static_cast<ToAsPointer>(&f);
115}
116
117template<typename To, typename From>
118inline To bit_cast(const From& from) {
119 GOOGLE_COMPILE_ASSERT(sizeof(From) == sizeof(To),
120 bit_cast_with_different_sizes);
121 To dest;
122 memcpy(&dest, &from, sizeof(dest));
123 return dest;
124}
125
126} // namespace internal
127
128// We made these internal so that they would show up as such in the docs,
129// but we don't want to stick "internal::" in front of them everywhere.
130using internal::implicit_cast;
131using internal::down_cast;
132using internal::bit_cast;
133
134} // namespace protobuf
135} // namespace google
136
137#include <google/protobuf/port_undef.inc>
138
139#endif // GOOGLE_PROTOBUF_CASTS_H__
140

source code of include/google/protobuf/stubs/casts.h