reinterp.cpp

Think in C++文中代码实现

// File from page 753 in "Thinking in C++" by Bruce Eckel
//////////////////////////////////////////////////
// From the compressed package ECKELT02.ZIP 4/11/95
// (Original ECKELT01.ZIP dated 2/21/95)
// Copyright (c) Bruce Eckel, 1995 
// Source code file from the book "Thinking in C++", 
// Prentice Hall, 1995, ISBN: 0-13-917709-4
// All rights reserved EXCEPT as allowed by the following 
// statements: You may freely use this file for your own 
// work, including modifications and distribution in 
// executable form only. You may copy and distribute this 
// file, as long as it is only distributed in the complete 
// (compressed) package with the other files from this 
// book and you do not remove this copyright and notice. 
// You may not distribute modified versions of the source 
// code in this package. This package may be freely placed 
// on bulletin boards, internet nodes, shareware disks and 
// product vendor disks. You may not use this file in 
// printed media without the express permission of the 
// author. Bruce Eckel makes no 
// representation about the suitability of this software 
// for any purpose. It is provided "as is" without express 
// or implied warranty of any kind. The entire risk as to 
// the quality and performance of the software is with 
// you. Should the software prove defective, you assume 
// the cost of all necessary servicing, repair, or 
// correction. 
// If you think you've found an error, please 
// email all modified files with loudly commented changes 
// to: [e-mail] (please use the same 
// address for non-code errors found in the book).
//////////////////////////////////////////////////

//: REINTERP.CPP -- Reinterpret_cast
// Example depends on VPTR location,
// Which may differ between compilers.
#include <string.h>
#include <fstream.h>
ofstream out("reinterp.out");

class X {
  enum { sz = 5 };
  int a[sz];
public:
  X() { memset(a, 0, sz * sizeof(int)); }
  virtual void f() {}
  // Size of all the data members:
  int membsize() { return sizeof(a); }
  friend ostream&
    operator<<(ostream& os, const X& x) {
      for(int i = 0; i < sz; i++)
        os << x.a[i] << ' ';
      return os;
  }
};

main() {
  X x;
  out << x << endl; // Initialized to zeroes
  int* xp = reinterpret_cast<int*>(&x);
  xp[1] = 47;
  out << x << endl; // Oops!

  X x2;
  const vptr_size = sizeof(X) - x2.membsize();
  long l = reinterpret_cast<long>(&x2);
  // *IF* the VPTR is first in the object:
  l += vptr_size; // Move past VPTR
  xp = reinterpret_cast<int*>(l);
  xp[1] = 47;
  out << x2 << endl;
}