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Diffstat (limited to 'chromium/base/macros.h')
| -rw-r--r-- | chromium/base/macros.h | 313 |
1 files changed, 313 insertions, 0 deletions
diff --git a/chromium/base/macros.h b/chromium/base/macros.h new file mode 100644 index 00000000000..781751569d7 --- /dev/null +++ b/chromium/base/macros.h @@ -0,0 +1,313 @@ +// Copyright 2014 The Chromium Authors. All rights reserved. +// Use of this source code is governed by a BSD-style license that can be +// found in the LICENSE file. + +// This file contains macros and macro-like constructs (e.g., templates) that +// are commonly used throughout Chromium source. (It may also contain things +// that are closely related to things that are commonly used that belong in this +// file.) + +#ifndef BASE_MACROS_H_ +#define BASE_MACROS_H_ + +#include <stddef.h> // For size_t. +#include <string.h> // For memcpy. + +#include "base/compiler_specific.h" // For ALLOW_UNUSED. + +// Put this in the private: declarations for a class to be uncopyable. +#define DISALLOW_COPY(TypeName) \ + TypeName(const TypeName&) + +// Put this in the private: declarations for a class to be unassignable. +#define DISALLOW_ASSIGN(TypeName) \ + void operator=(const TypeName&) + +// A macro to disallow the copy constructor and operator= functions +// This should be used in the private: declarations for a class +#define DISALLOW_COPY_AND_ASSIGN(TypeName) \ + TypeName(const TypeName&); \ + void operator=(const TypeName&) + +// An older, deprecated, politically incorrect name for the above. +// NOTE: The usage of this macro was banned from our code base, but some +// third_party libraries are yet using it. +// TODO(tfarina): Figure out how to fix the usage of this macro in the +// third_party libraries and get rid of it. +#define DISALLOW_EVIL_CONSTRUCTORS(TypeName) DISALLOW_COPY_AND_ASSIGN(TypeName) + +// A macro to disallow all the implicit constructors, namely the +// default constructor, copy constructor and operator= functions. +// +// This should be used in the private: declarations for a class +// that wants to prevent anyone from instantiating it. This is +// especially useful for classes containing only static methods. +#define DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName) \ + TypeName(); \ + DISALLOW_COPY_AND_ASSIGN(TypeName) + +// The arraysize(arr) macro returns the # of elements in an array arr. +// The expression is a compile-time constant, and therefore can be +// used in defining new arrays, for example. If you use arraysize on +// a pointer by mistake, you will get a compile-time error. +// +// One caveat is that arraysize() doesn't accept any array of an +// anonymous type or a type defined inside a function. In these rare +// cases, you have to use the unsafe ARRAYSIZE_UNSAFE() macro below. This is +// due to a limitation in C++'s template system. The limitation might +// eventually be removed, but it hasn't happened yet. + +// This template function declaration is used in defining arraysize. +// Note that the function doesn't need an implementation, as we only +// use its type. +template <typename T, size_t N> +char (&ArraySizeHelper(T (&array)[N]))[N]; + +// That gcc wants both of these prototypes seems mysterious. VC, for +// its part, can't decide which to use (another mystery). Matching of +// template overloads: the final frontier. +#ifndef _MSC_VER +template <typename T, size_t N> +char (&ArraySizeHelper(const T (&array)[N]))[N]; +#endif + +#define arraysize(array) (sizeof(ArraySizeHelper(array))) + +// ARRAYSIZE_UNSAFE performs essentially the same calculation as arraysize, +// but can be used on anonymous types or types defined inside +// functions. It's less safe than arraysize as it accepts some +// (although not all) pointers. Therefore, you should use arraysize +// whenever possible. +// +// The expression ARRAYSIZE_UNSAFE(a) is a compile-time constant of type +// size_t. +// +// ARRAYSIZE_UNSAFE catches a few type errors. If you see a compiler error +// +// "warning: division by zero in ..." +// +// when using ARRAYSIZE_UNSAFE, you are (wrongfully) giving it a pointer. +// You should only use ARRAYSIZE_UNSAFE on statically allocated arrays. +// +// The following comments are on the implementation details, and can +// be ignored by the users. +// +// ARRAYSIZE_UNSAFE(arr) works by inspecting sizeof(arr) (the # of bytes in +// the array) and sizeof(*(arr)) (the # of bytes in one array +// element). If the former is divisible by the latter, perhaps arr is +// indeed an array, in which case the division result is the # of +// elements in the array. Otherwise, arr cannot possibly be an array, +// and we generate a compiler error to prevent the code from +// compiling. +// +// Since the size of bool is implementation-defined, we need to cast +// !(sizeof(a) & sizeof(*(a))) to size_t in order to ensure the final +// result has type size_t. +// +// This macro is not perfect as it wrongfully accepts certain +// pointers, namely where the pointer size is divisible by the pointee +// size. Since all our code has to go through a 32-bit compiler, +// where a pointer is 4 bytes, this means all pointers to a type whose +// size is 3 or greater than 4 will be (righteously) rejected. + +#define ARRAYSIZE_UNSAFE(a) \ + ((sizeof(a) / sizeof(*(a))) / \ + static_cast<size_t>(!(sizeof(a) % sizeof(*(a))))) + + +// Use implicit_cast as a safe version of static_cast or const_cast +// for upcasting in the type hierarchy (i.e. casting a pointer to Foo +// to a pointer to SuperclassOfFoo or casting a pointer to Foo to +// a const pointer to Foo). +// When you use implicit_cast, the compiler checks that the cast is safe. +// Such explicit implicit_casts are necessary in surprisingly many +// situations where C++ demands an exact type match instead of an +// argument type convertible to a target type. +// +// The From type can be inferred, so the preferred syntax for using +// implicit_cast is the same as for static_cast etc.: +// +// implicit_cast<ToType>(expr) +// +// implicit_cast would have been part of the C++ standard library, +// but the proposal was submitted too late. It will probably make +// its way into the language in the future. +template<typename To, typename From> +inline To implicit_cast(From const &f) { + return f; +} + +// The COMPILE_ASSERT macro can be used to verify that a compile time +// expression is true. For example, you could use it to verify the +// size of a static array: +// +// COMPILE_ASSERT(ARRAYSIZE_UNSAFE(content_type_names) == CONTENT_NUM_TYPES, +// content_type_names_incorrect_size); +// +// or to make sure a struct is smaller than a certain size: +// +// COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large); +// +// The second argument to the macro is the name of the variable. If +// the expression is false, most compilers will issue a warning/error +// containing the name of the variable. + +#undef COMPILE_ASSERT + +#if __cplusplus >= 201103L + +// Under C++11, just use static_assert. +#define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg) + +#else + +template <bool> +struct CompileAssert { +}; + +#define COMPILE_ASSERT(expr, msg) \ + typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1] ALLOW_UNUSED + +// Implementation details of COMPILE_ASSERT: +// +// - COMPILE_ASSERT works by defining an array type that has -1 +// elements (and thus is invalid) when the expression is false. +// +// - The simpler definition +// +// #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1] +// +// does not work, as gcc supports variable-length arrays whose sizes +// are determined at run-time (this is gcc's extension and not part +// of the C++ standard). As a result, gcc fails to reject the +// following code with the simple definition: +// +// int foo; +// COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is +// // not a compile-time constant. +// +// - By using the type CompileAssert<(bool(expr))>, we ensures that +// expr is a compile-time constant. (Template arguments must be +// determined at compile-time.) +// +// - The outer parentheses in CompileAssert<(bool(expr))> are necessary +// to work around a bug in gcc 3.4.4 and 4.0.1. If we had written +// +// CompileAssert<bool(expr)> +// +// instead, these compilers will refuse to compile +// +// COMPILE_ASSERT(5 > 0, some_message); +// +// (They seem to think the ">" in "5 > 0" marks the end of the +// template argument list.) +// +// - The array size is (bool(expr) ? 1 : -1), instead of simply +// +// ((expr) ? 1 : -1). +// +// This is to avoid running into a bug in MS VC 7.1, which +// causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1. + +#endif + +// bit_cast<Dest,Source> is a template function that implements the +// equivalent of "*reinterpret_cast<Dest*>(&source)". We need this in +// very low-level functions like the protobuf library and fast math +// support. +// +// float f = 3.14159265358979; +// int i = bit_cast<int32>(f); +// // i = 0x40490fdb +// +// The classical address-casting method is: +// +// // WRONG +// float f = 3.14159265358979; // WRONG +// int i = * reinterpret_cast<int*>(&f); // WRONG +// +// The address-casting method actually produces undefined behavior +// according to ISO C++ specification section 3.10 -15 -. Roughly, this +// section says: if an object in memory has one type, and a program +// accesses it with a different type, then the result is undefined +// behavior for most values of "different type". +// +// This is true for any cast syntax, either *(int*)&f or +// *reinterpret_cast<int*>(&f). And it is particularly true for +// conversions between integral lvalues and floating-point lvalues. +// +// The purpose of 3.10 -15- is to allow optimizing compilers to assume +// that expressions with different types refer to different memory. gcc +// 4.0.1 has an optimizer that takes advantage of this. So a +// non-conforming program quietly produces wildly incorrect output. +// +// The problem is not the use of reinterpret_cast. The problem is type +// punning: holding an object in memory of one type and reading its bits +// back using a different type. +// +// The C++ standard is more subtle and complex than this, but that +// is the basic idea. +// +// Anyways ... +// +// bit_cast<> calls memcpy() which is blessed by the standard, +// especially by the example in section 3.9 . Also, of course, +// bit_cast<> wraps up the nasty logic in one place. +// +// Fortunately memcpy() is very fast. In optimized mode, with a +// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline +// code with the minimal amount of data movement. On a 32-bit system, +// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8) +// compiles to two loads and two stores. +// +// I tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc 7.1. +// +// WARNING: if Dest or Source is a non-POD type, the result of the memcpy +// is likely to surprise you. + +template <class Dest, class Source> +inline Dest bit_cast(const Source& source) { + COMPILE_ASSERT(sizeof(Dest) == sizeof(Source), VerifySizesAreEqual); + + Dest dest; + memcpy(&dest, &source, sizeof(dest)); + return dest; +} + +// Used to explicitly mark the return value of a function as unused. If you are +// really sure you don't want to do anything with the return value of a function +// that has been marked WARN_UNUSED_RESULT, wrap it with this. Example: +// +// scoped_ptr<MyType> my_var = ...; +// if (TakeOwnership(my_var.get()) == SUCCESS) +// ignore_result(my_var.release()); +// +template<typename T> +inline void ignore_result(const T&) { +} + +// The following enum should be used only as a constructor argument to indicate +// that the variable has static storage class, and that the constructor should +// do nothing to its state. It indicates to the reader that it is legal to +// declare a static instance of the class, provided the constructor is given +// the base::LINKER_INITIALIZED argument. Normally, it is unsafe to declare a +// static variable that has a constructor or a destructor because invocation +// order is undefined. However, IF the type can be initialized by filling with +// zeroes (which the loader does for static variables), AND the destructor also +// does nothing to the storage, AND there are no virtual methods, then a +// constructor declared as +// explicit MyClass(base::LinkerInitialized x) {} +// and invoked as +// static MyClass my_variable_name(base::LINKER_INITIALIZED); +namespace base { +enum LinkerInitialized { LINKER_INITIALIZED }; + +// Use these to declare and define a static local variable (static T;) so that +// it is leaked so that its destructors are not called at exit. If you need +// thread-safe initialization, use base/lazy_instance.h instead. +#define CR_DEFINE_STATIC_LOCAL(type, name, arguments) \ + static type& name = *new type arguments + +} // base + +#endif // BASE_MACROS_H_ |