1// Copyright (c) 2009 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// Various Google-specific macros. 6// 7// This code is compiled directly on many platforms, including client 8// platforms like Windows, Mac, and embedded systems. Before making 9// any changes here, make sure that you're not breaking any platforms. 10// 11 12#ifndef BASE_MACROS_H_ 13#define BASE_MACROS_H_ 14 15#include <stddef.h> // For size_t 16 17#include "base/type_traits.h" 18 19 20// The COMPILE_ASSERT macro can be used to verify that a compile time 21// expression is true. For example, you could use it to verify the 22// size of a static array: 23// 24// COMPILE_ASSERT(ARRAYSIZE(content_type_names) == CONTENT_NUM_TYPES, 25// content_type_names_incorrect_size); 26// 27// or to make sure a struct is smaller than a certain size: 28// 29// COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large); 30// 31// The second argument to the macro is the name of the variable. If 32// the expression is false, most compilers will issue a warning/error 33// containing the name of the variable. 34 35#define COMPILE_ASSERT(expr, msg) \ 36 typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1] 37 38// Implementation details of COMPILE_ASSERT: 39// 40// - COMPILE_ASSERT works by defining an array type that has -1 41// elements (and thus is invalid) when the expression is false. 42// 43// - The simpler definition 44// 45// #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1] 46// 47// does not work, as gcc supports variable-length arrays whose sizes 48// are determined at run-time (this is gcc's extension and not part 49// of the C++ standard). As a result, gcc fails to reject the 50// following code with the simple definition: 51// 52// int foo; 53// COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is 54// // not a compile-time constant. 55// 56// - By using the type CompileAssert<(bool(expr))>, we ensures that 57// expr is a compile-time constant. (Template arguments must be 58// determined at compile-time.) 59// 60// - The outter parentheses in CompileAssert<(bool(expr))> are necessary 61// to work around a bug in gcc 3.4.4 and 4.0.1. If we had written 62// 63// CompileAssert<bool(expr)> 64// 65// instead, these compilers will refuse to compile 66// 67// COMPILE_ASSERT(5 > 0, some_message); 68// 69// (They seem to think the ">" in "5 > 0" marks the end of the 70// template argument list.) 71// 72// - The array size is (bool(expr) ? 1 : -1), instead of simply 73// 74// ((expr) ? 1 : -1). 75// 76// This is to avoid running into a bug in MS VC 7.1, which 77// causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1. 78 79 80// A macro to disallow the copy constructor and operator= functions 81// This should be used in the private: declarations for a class 82// 83// For disallowing only assign or copy, write the code directly, but declare 84// the intend in a comment, for example: 85// void operator=(const TypeName&); // DISALLOW_ASSIGN 86// Note, that most uses of DISALLOW_ASSIGN and DISALLOW_COPY are broken 87// semantically, one should either use disallow both or neither. Try to 88// avoid these in new code. 89#define DISALLOW_COPY_AND_ASSIGN(TypeName) \ 90 TypeName(const TypeName&); \ 91 void operator=(const TypeName&) 92 93// An older, politically incorrect name for the above. 94// Prefer DISALLOW_COPY_AND_ASSIGN for new code. 95#define DISALLOW_EVIL_CONSTRUCTORS(TypeName) DISALLOW_COPY_AND_ASSIGN(TypeName) 96 97// A macro to disallow all the implicit constructors, namely the 98// default constructor, copy constructor and operator= functions. 99// 100// This should be used in the private: declarations for a class 101// that wants to prevent anyone from instantiating it. This is 102// especially useful for classes containing only static methods. 103#define DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName) \ 104 TypeName(); \ 105 DISALLOW_COPY_AND_ASSIGN(TypeName) 106 107// The arraysize(arr) macro returns the # of elements in an array arr. 108// The expression is a compile-time constant, and therefore can be 109// used in defining new arrays, for example. If you use arraysize on 110// a pointer by mistake, you will get a compile-time error. 111// 112// One caveat is that arraysize() doesn't accept any array of an 113// anonymous type or a type defined inside a function. In these rare 114// cases, you have to use the unsafe ARRAYSIZE() macro below. This is 115// due to a limitation in C++'s template system. The limitation might 116// eventually be removed, but it hasn't happened yet. 117 118// This template function declaration is used in defining arraysize. 119// Note that the function doesn't need an implementation, as we only 120// use its type. 121template <typename T, size_t N> 122char (&ArraySizeHelper(T (&array)[N]))[N]; 123 124// That gcc wants both of these prototypes seems mysterious. VC, for 125// its part, can't decide which to use (another mystery). Matching of 126// template overloads: the final frontier. 127#ifndef COMPILER_MSVC 128template <typename T, size_t N> 129char (&ArraySizeHelper(const T (&array)[N]))[N]; 130#endif 131 132#define arraysize(array) (sizeof(ArraySizeHelper(array))) 133 134// ARRAYSIZE performs essentially the same calculation as arraysize, 135// but can be used on anonymous types or types defined inside 136// functions. It's less safe than arraysize as it accepts some 137// (although not all) pointers. Therefore, you should use arraysize 138// whenever possible. 139// 140// The expression ARRAYSIZE(a) is a compile-time constant of type 141// size_t. 142// 143// ARRAYSIZE catches a few type errors. If you see a compiler error 144// 145// "warning: division by zero in ..." 146// 147// when using ARRAYSIZE, you are (wrongfully) giving it a pointer. 148// You should only use ARRAYSIZE on statically allocated arrays. 149// 150// The following comments are on the implementation details, and can 151// be ignored by the users. 152// 153// ARRAYSIZE(arr) works by inspecting sizeof(arr) (the # of bytes in 154// the array) and sizeof(*(arr)) (the # of bytes in one array 155// element). If the former is divisible by the latter, perhaps arr is 156// indeed an array, in which case the division result is the # of 157// elements in the array. Otherwise, arr cannot possibly be an array, 158// and we generate a compiler error to prevent the code from 159// compiling. 160// 161// Since the size of bool is implementation-defined, we need to cast 162// !(sizeof(a) & sizeof(*(a))) to size_t in order to ensure the final 163// result has type size_t. 164// 165// This macro is not perfect as it wrongfully accepts certain 166// pointers, namely where the pointer size is divisible by the pointee 167// size. Since all our code has to go through a 32-bit compiler, 168// where a pointer is 4 bytes, this means all pointers to a type whose 169// size is 3 or greater than 4 will be (righteously) rejected. 170// 171// Kudos to Jorg Brown for this simple and elegant implementation. 172// 173// - wan 2005-11-16 174// 175// Starting with Visual C++ 2005, WinNT.h includes ARRAYSIZE. 176#if !defined(COMPILER_MSVC) || (defined(_MSC_VER) && _MSC_VER < 1400) 177#define ARRAYSIZE(a) \ 178 ((sizeof(a) / sizeof(*(a))) / \ 179 static_cast<size_t>(!(sizeof(a) % sizeof(*(a))))) 180#endif 181 182// A macro to turn a symbol into a string 183#define AS_STRING(x) AS_STRING_INTERNAL(x) 184#define AS_STRING_INTERNAL(x) #x 185 186 187// One of the type traits, is_pod, makes it possible to query whether 188// a type is a POD type. It is impossible for type_traits.h to get 189// this right without compiler support, so it fails conservatively. It 190// knows that fundamental types and pointers are PODs, but it can't 191// tell whether user classes are PODs. The DECLARE_POD macro is used 192// to inform the type traits library that a user class is a POD. 193// 194// Implementation note: the typedef at the end is just to make it legal 195// to put a semicolon after DECLARE_POD(foo). 196// 197// 198// So what's a POD? The C++ standard (clause 9 paragraph 4) gives a 199// full definition, but a good rule of thumb is that a struct is a POD 200// ("plain old data") if it doesn't use any of the features that make 201// C++ different from C. A POD struct can't have constructors, 202// destructors, assignment operators, base classes, private or 203// protected members, or virtual functions, and all of its member 204// variables must themselves be PODs. 205 206#define DECLARE_POD(TypeName) \ 207namespace base { \ 208template<> struct is_pod<TypeName> : true_type { }; \ 209} \ 210typedef int Dummy_Type_For_DECLARE_POD \ 211 212// We once needed a different technique to assert that a nested class 213// is a POD. This is no longer necessary, and DECLARE_NESTED_POD is 214// just a synonym for DECLARE_POD. We continue to provide 215// DECLARE_NESTED_POD only so we don't have to change client 216// code. Regardless of whether you use DECLARE_POD or 217// DECLARE_NESTED_POD: use it after the outer class. Using it within a 218// class definition will give a compiler error. 219#define DECLARE_NESTED_POD(TypeName) DECLARE_POD(TypeName) 220 221// Declare that TemplateName<T> is a POD whenever T is 222#define PROPAGATE_POD_FROM_TEMPLATE_ARGUMENT(TemplateName) \ 223namespace base { \ 224template <typename T> struct is_pod<TemplateName<T> > : is_pod<T> { }; \ 225} \ 226typedef int Dummy_Type_For_PROPAGATE_POD_FROM_TEMPLATE_ARGUMENT 227 228// Macro that does nothing if TypeName is a POD, and gives a compiler 229// error if TypeName is a non-POD. You should put a descriptive 230// comment right next to the macro call so that people can tell what 231// the compiler error is about. 232// 233// Implementation note: this works by taking the size of a type that's 234// complete when TypeName is a POD and incomplete otherwise. 235 236template <typename Boolean> struct ERROR_TYPE_MUST_BE_POD; 237template <> struct ERROR_TYPE_MUST_BE_POD<base::true_type> { }; 238#define ENFORCE_POD(TypeName) \ 239 enum { dummy_##TypeName \ 240 = sizeof(ERROR_TYPE_MUST_BE_POD< \ 241 typename base::is_pod<TypeName>::type>) } 242 243#endif // BASE_MACROS_H_ 244