reinterp.cpp

Thinking in C++ ２.０书籍源码光盘

//: C24:Reinterp.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 1999
// Copyright notice in Copyright.txt
// Reinterpret_cast
// Example depends on VPTR location,
// Which may differ between compilers.
#include <cstring>
#include <fstream>
using namespace std;
ofstream out("reinterp.out");

class X {
  static const int 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;
  }
  virtual ~X() {}
};

int 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 int 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;
} ///:~