//: C12:ReferenceCounting.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
// Reference count, copy-on-write
#include "../require.h"
#include <string>
#include <iostream>
using namespace std;
class Dog {
string nm;
int refcount;
Dog(const string& name)
: nm(name), refcount(1) {
cout << "Creating Dog: " << *this << endl;
}
// Prevent assignment:
Dog& operator=(const Dog& rv);
public:
// Dogs can only be created on the heap:
static Dog* make(const string& name) {
return new Dog(name);
}
Dog(const Dog& d)
: nm(d.nm + " copy"), refcount(1) {
cout << "Dog copy-constructor: "
<< *this << endl;
}
~Dog() {
cout << "Deleting Dog: " << *this << endl;
}
void attach() {
++refcount;
cout << "Attached Dog: " << *this << endl;
}
void detach() {
require(refcount != 0);
cout << "Detaching Dog: " << *this << endl;
// Destroy object if no one is using it:
if(--refcount == 0) delete this;
}
// Conditionally copy this Dog.
// Call before modifying the Dog, assign
// resulting pointer to your Dog*.
Dog* unalias() {
cout << "Unaliasing Dog: " << *this << endl;
// Don't duplicate if not aliased:
if(refcount == 1) return this;
--refcount;
// Use copy-constructor to duplicate:
return new Dog(*this);
}
void rename(const string& newName) {
nm = newName;
cout << "Dog renamed to: " << *this << endl;
}
friend ostream&
operator<<(ostream& os, const Dog& d) {
return os << "[" << d.nm << "], rc = "
<< d.refcount;
}
};
class DogHouse {
Dog* p;
string houseName;
public:
DogHouse(Dog* dog, const string& house)
: p(dog), houseName(house) {
cout << "Created DogHouse: "<< *this << endl;
}
DogHouse(const DogHouse& dh)
: p(dh.p),
houseName("copy-constructed " +
dh.houseName) {
p->attach();
cout << "DogHouse copy-constructor: "
<< *this << endl;
}
DogHouse& operator=(const DogHouse& dh) {
// Check for self-assignment:
if(&dh != this) {
houseName = dh.houseName + " assigned";
// Clean up what you're using first:
p->detach();
p = dh.p; // Like copy-constructor
p->attach();
}
cout << "DogHouse operator= : "
<< *this << endl;
return *this;
}
// Decrement refcount, conditionally destroy
~DogHouse() {
cout << "DogHouse destructor: "
<< *this << endl;
p->detach();
}
void renameHouse(const string& newName) {
houseName = newName;
}
void unalias() { p = p->unalias(); }
// Copy-on-write. Anytime you modify the
// contents of the pointer you must
// first unalias it:
void renameDog(const string& newName) {
unalias();
p->rename(newName);
}
// ... or when you allow someone else access:
Dog* getDog() {
unalias();
return p;
}
friend ostream&
operator<<(ostream& os, const DogHouse& dh) {
return os << "[" << dh.houseName
<< "] contains " << *dh.p;
}
};
int main() {
DogHouse
fidos(Dog::make("Fido"), "FidoHouse"),
spots(Dog::make("Spot"), "SpotHouse");
cout << "Entering copy-construction" << endl;
DogHouse bobs(fidos);
cout << "After copy-constructing bobs" << endl;
cout << "fidos:" << fidos << endl;
cout << "spots:" << spots << endl;
cout << "bobs:" << bobs << endl;
cout << "Entering spots = fidos" << endl;
spots = fidos;
cout << "After spots = fidos" << endl;
cout << "spots:" << spots << endl;
cout << "Entering self-assignment" << endl;
bobs = bobs;
cout << "After self-assignment" << endl;
cout << "bobs:" << bobs << endl;
// Comment out the following lines:
cout << "Entering rename(\"Bob\")" << endl;
bobs.getDog()->rename("Bob");
cout << "After rename(\"Bob\")" << endl;
} ///:~
Please read the code,and continue only after you understand it,it's pretty hard to explain it by writing.
So,there are 2 classes,doghouse and dog.Multiple doghouses can be attached to a dog,that's the main ideea.
Now,let's focus on this:
p->detach();
p = dh.p; // Like copy-constructor
p->attach();
this line: p = dh.p
creates a dog with a refcount of 1,which is fine but later attach is called and the refcount goes to 2.Why 2? only a doghouse is really attached to it!
If you understand the code,please explain with details when I assign a doghouse to another(and they're both initialized ofcourse)