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