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The CICA Windows Explosion!
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10
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REFCOUNT.CPP
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1995-02-23
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106 lines
// File from page 436 in "Thinking in C++" by Bruce Eckel
//////////////////////////////////////////////////
// From the compressed package ECKELT01.ZIP 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: eckel@aol.com (please use the same
// address for non-code errors found in the book).
//////////////////////////////////////////////////
//: REFCOUNT.CPP -- Reference count, copy-on-write
#include <string.h>
#include <assert.h>
class counted {
class memblock {
enum { size = 100 };
char c[size];
int refcount;
public:
memblock() {
memset(c, 1, size);
refcount = 1;
}
memblock(const memblock& rv) {
memcpy(c, rv.c, size);
refcount = 1;
}
void attach() { ++refcount; }
void detach() {
assert(refcount != 0);
// Destroy object if no one is using it:
if(--refcount == 0) delete this;
}
int count() const { return refcount; }
void set(char x) { memset(c, x, size); }
// Conditionally copy this memblock.
// Call before modifying the block; assign
// resulting pointer to your block;
memblock* unalias() {
// Don't duplicate if not aliased:
if(refcount == 1) return this;
--refcount;
// Use copy-constructor to duplicate:
return new memblock(*this);
}
} * block;
public:
counted() {
block = new memblock; // Sneak preview
}
counted(const counted& rv) {
block = rv.block; // Pointer assignment
block->attach();
}
void unalias() { block = block->unalias(); }
counted& operator=(const counted& rv) {
// Check for self-assignment:
if(&rv == this) return *this;
// Clean up what you're using first:
block->detach();
block = rv.block; // Like copy-constructor
block->attach();
return *this;
}
// Decrement refcount, conditionally destroy
~counted() { block->detach(); }
// Copy-on-write:
void write(char value) {
// Do this before any write operation:
unalias();
// It's safe to write now.
block->set(value);
}
};
main() {
counted A, B;
counted C(A);
B = A;
C = C;
C.write('x');
}