1 // Copyright 2014 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // This file contains macros and macro-like constructs (e.g., templates) that 6 // are commonly used throughout Chromium source. (It may also contain things 7 // that are closely related to things that are commonly used that belong in this 8 // file.) 9 10 #ifndef BASE_MACROS_H_ 11 #define BASE_MACROS_H_ 12 13 #include <stddef.h> // For size_t. 14 #include <string.h> // For memcpy. 15 16 #include "base/compiler_specific.h" // For ALLOW_UNUSED. 17 18 // Put this in the private: declarations for a class to be uncopyable. 19 #define DISALLOW_COPY(TypeName) \ 20 TypeName(const TypeName&) 21 22 // Put this in the private: declarations for a class to be unassignable. 23 #define DISALLOW_ASSIGN(TypeName) \ 24 void operator=(const TypeName&) 25 26 // A macro to disallow the copy constructor and operator= functions 27 // This should be used in the private: declarations for a class 28 #define DISALLOW_COPY_AND_ASSIGN(TypeName) \ 29 TypeName(const TypeName&); \ 30 void operator=(const TypeName&) 31 32 // An older, deprecated, politically incorrect name for the above. 33 // NOTE: The usage of this macro was banned from our code base, but some 34 // third_party libraries are yet using it. 35 // TODO(tfarina): Figure out how to fix the usage of this macro in the 36 // third_party libraries and get rid of it. 37 #define DISALLOW_EVIL_CONSTRUCTORS(TypeName) DISALLOW_COPY_AND_ASSIGN(TypeName) 38 39 // A macro to disallow all the implicit constructors, namely the 40 // default constructor, copy constructor and operator= functions. 41 // 42 // This should be used in the private: declarations for a class 43 // that wants to prevent anyone from instantiating it. This is 44 // especially useful for classes containing only static methods. 45 #define DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName) \ 46 TypeName(); \ 47 DISALLOW_COPY_AND_ASSIGN(TypeName) 48 49 // The arraysize(arr) macro returns the # of elements in an array arr. 50 // The expression is a compile-time constant, and therefore can be 51 // used in defining new arrays, for example. If you use arraysize on 52 // a pointer by mistake, you will get a compile-time error. 53 // 54 // One caveat is that arraysize() doesn't accept any array of an 55 // anonymous type or a type defined inside a function. In these rare 56 // cases, you have to use the unsafe ARRAYSIZE_UNSAFE() macro below. This is 57 // due to a limitation in C++'s template system. The limitation might 58 // eventually be removed, but it hasn't happened yet. 59 60 // This template function declaration is used in defining arraysize. 61 // Note that the function doesn't need an implementation, as we only 62 // use its type. 63 template <typename T, size_t N> 64 char (&ArraySizeHelper(T (&array)[N]))[N]; 65 66 // That gcc wants both of these prototypes seems mysterious. VC, for 67 // its part, can't decide which to use (another mystery). Matching of 68 // template overloads: the final frontier. 69 #ifndef _MSC_VER 70 template <typename T, size_t N> 71 char (&ArraySizeHelper(const T (&array)[N]))[N]; 72 #endif 73 74 #define arraysize(array) (sizeof(ArraySizeHelper(array))) 75 76 // ARRAYSIZE_UNSAFE performs essentially the same calculation as arraysize, 77 // but can be used on anonymous types or types defined inside 78 // functions. It's less safe than arraysize as it accepts some 79 // (although not all) pointers. Therefore, you should use arraysize 80 // whenever possible. 81 // 82 // The expression ARRAYSIZE_UNSAFE(a) is a compile-time constant of type 83 // size_t. 84 // 85 // ARRAYSIZE_UNSAFE catches a few type errors. If you see a compiler error 86 // 87 // "warning: division by zero in ..." 88 // 89 // when using ARRAYSIZE_UNSAFE, you are (wrongfully) giving it a pointer. 90 // You should only use ARRAYSIZE_UNSAFE on statically allocated arrays. 91 // 92 // The following comments are on the implementation details, and can 93 // be ignored by the users. 94 // 95 // ARRAYSIZE_UNSAFE(arr) works by inspecting sizeof(arr) (the # of bytes in 96 // the array) and sizeof(*(arr)) (the # of bytes in one array 97 // element). If the former is divisible by the latter, perhaps arr is 98 // indeed an array, in which case the division result is the # of 99 // elements in the array. Otherwise, arr cannot possibly be an array, 100 // and we generate a compiler error to prevent the code from 101 // compiling. 102 // 103 // Since the size of bool is implementation-defined, we need to cast 104 // !(sizeof(a) & sizeof(*(a))) to size_t in order to ensure the final 105 // result has type size_t. 106 // 107 // This macro is not perfect as it wrongfully accepts certain 108 // pointers, namely where the pointer size is divisible by the pointee 109 // size. Since all our code has to go through a 32-bit compiler, 110 // where a pointer is 4 bytes, this means all pointers to a type whose 111 // size is 3 or greater than 4 will be (righteously) rejected. 112 113 #define ARRAYSIZE_UNSAFE(a) \ 114 ((sizeof(a) / sizeof(*(a))) / \ 115 static_cast<size_t>(!(sizeof(a) % sizeof(*(a))))) 116 117 118 // Use implicit_cast as a safe version of static_cast or const_cast 119 // for upcasting in the type hierarchy (i.e. casting a pointer to Foo 120 // to a pointer to SuperclassOfFoo or casting a pointer to Foo to 121 // a const pointer to Foo). 122 // When you use implicit_cast, the compiler checks that the cast is safe. 123 // Such explicit implicit_casts are necessary in surprisingly many 124 // situations where C++ demands an exact type match instead of an 125 // argument type convertible to a target type. 126 // 127 // The From type can be inferred, so the preferred syntax for using 128 // implicit_cast is the same as for static_cast etc.: 129 // 130 // implicit_cast<ToType>(expr) 131 // 132 // implicit_cast would have been part of the C++ standard library, 133 // but the proposal was submitted too late. It will probably make 134 // its way into the language in the future. 135 template<typename To, typename From> 136 inline To implicit_cast(From const &f) { 137 return f; 138 } 139 140 // The COMPILE_ASSERT macro can be used to verify that a compile time 141 // expression is true. For example, you could use it to verify the 142 // size of a static array: 143 // 144 // COMPILE_ASSERT(ARRAYSIZE_UNSAFE(content_type_names) == CONTENT_NUM_TYPES, 145 // content_type_names_incorrect_size); 146 // 147 // or to make sure a struct is smaller than a certain size: 148 // 149 // COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large); 150 // 151 // The second argument to the macro is the name of the variable. If 152 // the expression is false, most compilers will issue a warning/error 153 // containing the name of the variable. 154 155 #undef COMPILE_ASSERT 156 #define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg) 157 158 // bit_cast<Dest,Source> is a template function that implements the 159 // equivalent of "*reinterpret_cast<Dest*>(&source)". We need this in 160 // very low-level functions like the protobuf library and fast math 161 // support. 162 // 163 // float f = 3.14159265358979; 164 // int i = bit_cast<int32>(f); 165 // // i = 0x40490fdb 166 // 167 // The classical address-casting method is: 168 // 169 // // WRONG 170 // float f = 3.14159265358979; // WRONG 171 // int i = * reinterpret_cast<int*>(&f); // WRONG 172 // 173 // The address-casting method actually produces undefined behavior 174 // according to ISO C++ specification section 3.10 -15 -. Roughly, this 175 // section says: if an object in memory has one type, and a program 176 // accesses it with a different type, then the result is undefined 177 // behavior for most values of "different type". 178 // 179 // This is true for any cast syntax, either *(int*)&f or 180 // *reinterpret_cast<int*>(&f). And it is particularly true for 181 // conversions between integral lvalues and floating-point lvalues. 182 // 183 // The purpose of 3.10 -15- is to allow optimizing compilers to assume 184 // that expressions with different types refer to different memory. gcc 185 // 4.0.1 has an optimizer that takes advantage of this. So a 186 // non-conforming program quietly produces wildly incorrect output. 187 // 188 // The problem is not the use of reinterpret_cast. The problem is type 189 // punning: holding an object in memory of one type and reading its bits 190 // back using a different type. 191 // 192 // The C++ standard is more subtle and complex than this, but that 193 // is the basic idea. 194 // 195 // Anyways ... 196 // 197 // bit_cast<> calls memcpy() which is blessed by the standard, 198 // especially by the example in section 3.9 . Also, of course, 199 // bit_cast<> wraps up the nasty logic in one place. 200 // 201 // Fortunately memcpy() is very fast. In optimized mode, with a 202 // constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline 203 // code with the minimal amount of data movement. On a 32-bit system, 204 // memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8) 205 // compiles to two loads and two stores. 206 // 207 // I tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc 7.1. 208 // 209 // WARNING: if Dest or Source is a non-POD type, the result of the memcpy 210 // is likely to surprise you. 211 212 template <class Dest, class Source> 213 inline Dest bit_cast(const Source& source) { 214 COMPILE_ASSERT(sizeof(Dest) == sizeof(Source), VerifySizesAreEqual); 215 216 Dest dest; 217 memcpy(&dest, &source, sizeof(dest)); 218 return dest; 219 } 220 221 // Used to explicitly mark the return value of a function as unused. If you are 222 // really sure you don't want to do anything with the return value of a function 223 // that has been marked WARN_UNUSED_RESULT, wrap it with this. Example: 224 // 225 // scoped_ptr<MyType> my_var = ...; 226 // if (TakeOwnership(my_var.get()) == SUCCESS) 227 // ignore_result(my_var.release()); 228 // 229 template<typename T> 230 inline void ignore_result(const T&) { 231 } 232 233 // The following enum should be used only as a constructor argument to indicate 234 // that the variable has static storage class, and that the constructor should 235 // do nothing to its state. It indicates to the reader that it is legal to 236 // declare a static instance of the class, provided the constructor is given 237 // the base::LINKER_INITIALIZED argument. Normally, it is unsafe to declare a 238 // static variable that has a constructor or a destructor because invocation 239 // order is undefined. However, IF the type can be initialized by filling with 240 // zeroes (which the loader does for static variables), AND the destructor also 241 // does nothing to the storage, AND there are no virtual methods, then a 242 // constructor declared as 243 // explicit MyClass(base::LinkerInitialized x) {} 244 // and invoked as 245 // static MyClass my_variable_name(base::LINKER_INITIALIZED); 246 namespace base { 247 enum LinkerInitialized { LINKER_INITIALIZED }; 248 249 // Use these to declare and define a static local variable (static T;) so that 250 // it is leaked so that its destructors are not called at exit. If you need 251 // thread-safe initialization, use base/lazy_instance.h instead. 252 #define CR_DEFINE_STATIC_LOCAL(type, name, arguments) \ 253 static type& name = *new type arguments 254 255 } // base 256 257 #endif // BASE_MACROS_H_ 258
