base/macros.h

// 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&)

// 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.  For the technical details, refer to
// http://blogs.msdn.com/b/the1/archive/2004/05/07/128242.aspx.

// 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)))

// 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_t>(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) {
  static_assert(sizeof(Dest) == sizeof(Source),
                "bit_cast requires source and destination to be the same size");

  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_