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// 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.
// Put this in the declarations for a class to be uncopyable.
#define DISALLOW_COPY(TypeName) \
TypeName(const TypeName&) = delete
// Put this in the declarations for a class to be unassignable.
#define DISALLOW_ASSIGN(TypeName) \
void operator=(const TypeName&) = delete
// 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() = delete; \
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.
// 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];
#define arraysize(array) (sizeof(ArraySizeHelper(array)))
// 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(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
#define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg)
// 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_
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