volatile.cpp

Think in C++文中代码实现

// File from page 300 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 
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// 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).
//////////////////////////////////////////////////

//: VOLATILE.CPP -- The volatile keyword

class comm {
  const volatile unsigned char byte;
  volatile unsigned char flag;
  enum { bufsize = 100 };
  unsigned char buf[bufsize];
  int index;
public:
  comm();
  void isr() volatile;
  char read(int Index) const;
};

comm::comm() : index(0), byte(0), flag(0) {}

// Only a demo; won't actually work
// As an interrupt service routine:
void comm::isr() volatile {
  if(flag) flag = 0;
  buf[index++] = byte;
  // Wrap to beginning of buffer:
  if(index >= bufsize) index = 0;
}

char comm::read(int Index) const {
  if(Index < 0 || Index >= bufsize)
    return 0;
  return buf[Index];
}

main() {
  volatile comm Port;
  Port.isr(); // OK
//!  Port.read(0); // Not OK;
                // read() not volatile
}