mirror of
https://github.com/klzgrad/naiveproxy.git
synced 2024-11-28 00:06:09 +03:00
78 lines
3.0 KiB
C++
78 lines
3.0 KiB
C++
// Copyright 2016 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.
|
|
|
|
#ifndef BASE_BIT_CAST_H_
|
|
#define BASE_BIT_CAST_H_
|
|
|
|
#include <string.h>
|
|
#include <type_traits>
|
|
|
|
#include "base/compiler_specific.h"
|
|
#include "base/template_util.h"
|
|
#include "build/build_config.h"
|
|
|
|
// 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
|
|
// the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15.
|
|
// (This did not substantially change in C++11.) 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 this paragraph is to allow optimizing compilers to assume that
|
|
// expressions with different types refer to different memory. Compilers are
|
|
// known to take 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, compilers replace
|
|
// calls to memcpy() with inline object code when the size argument is a
|
|
// compile-time constant. 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.
|
|
|
|
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");
|
|
static_assert(base::is_trivially_copyable<Dest>::value,
|
|
"bit_cast requires the destination type to be copyable");
|
|
static_assert(base::is_trivially_copyable<Source>::value,
|
|
"bit_cast requires the source type to be copyable");
|
|
|
|
Dest dest;
|
|
memcpy(&dest, &source, sizeof(dest));
|
|
return dest;
|
|
}
|
|
|
|
#endif // BASE_BIT_CAST_H_
|