bit_cast.h

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/* +------------------------------------------------------------------------+
   |                     Mobile Robot Programming Toolkit (MRPT)            |
   |                          https://www.mrpt.org/                         |
   |                                                                        |
   | Copyright (c) 2005-2017, Individual contributors, see AUTHORS file     |
   | See: https://www.mrpt.org/Authors - All rights reserved.               |
   | Released under BSD License. See: https://www.mrpt.org/License          |
   +------------------------------------------------------------------------+ */

// 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.
#pragma once
#include <cstring>
#include <type_traits>
//#include "base/compiler_specific.h"  // mrpt
//#include "base/template_util.h"  // mrpt
//#include "build/build_config.h"  // mrpt
// 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(
        std::is_trivially_copyable<Dest>::value,
        "bit_cast requires the destination type to be copyable");
    static_assert(
        std::is_trivially_copyable<Source>::value,
        "bit_cast requires the source type to be copyable");
    Dest dest;
    memcpy(&dest, &source, sizeof(dest));
    return dest;
}