howmany2.cpp

Think in C++文中代码实现

// File from page 379 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).
//////////////////////////////////////////////////

//: HOWMANY2.CPP -- The copy-constructor
#include <fstream.h>
#include <string.h>
ofstream out("howmany2.out");

class howmany2 {
  enum { bufsize = 30 };
  char id[bufsize]; // Object identifier
  static int object_count;
public:
  howmany2(const char* ID = 0) {
    if(ID) strncpy(id, ID, bufsize);
    else *id = 0;
    ++object_count;
    print("howmany2()");
  }
  // The copy-constructor:
  howmany2(const howmany2& h) {
    strncpy(id, h.id, bufsize);
    strncat(id, " copy", bufsize - strlen(id));
    ++object_count;
    print("howmany2(howmany2&)");
  }
  // Can't be static (printing id):
  void print(const char* msg = 0) const {
    if(msg) out << msg << endl;
    out << '\t' << id << ": "
        << "object_count = "
        << object_count << endl;
  }
  ~howmany2() {
    --object_count;
    print("~howmany2()");
  }
};

int howmany2::object_count = 0;

// Pass and return BY VALUE:
howmany2 f(howmany2 x) {
  x.print("x argument inside f()");
  out << "returning from f()" << endl;
  return x;
}

main() {
  howmany2 h("h");
  out << "entering f()" << endl;
  howmany2 h2 = f(h);
  h2.print("h2 after call to f()");
  out << "call f(), no return value" << endl;
  f(h);
  out << "after call to f()" << endl;
}