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Written by Hemanshu Patel
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Monday, 22 October 2007 |
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Page 7 of 9 Reference Counting In the example above, the copy-constructor and operator= make a new copy of what the pointer points to, and the destructor deletes it. However, if your object requires a lot of memory or a high initialization overhead, you may want to avoid this copying. A common approach to this problem is called reference counting. You give intelligence to the object that’s being pointed to so it knows how many objects are pointing to it. Then copy-construction or assignment means attaching another pointer to an existing object and incrementing the reference count. Destruction means reducing the reference count and destroying the object if the reference count goes to zero. But what if you want to write to the object (the Dog in the example above)? More than one object may be using this Dog, so you’d be modifying someone else’s Dog as well as yours, which doesn’t seem very neighborly. To solve this “aliasing” problem, an additional technique called copy-on-write is used. Before writing to a block of memory, you make sure no one else is using it. If the reference count is greater than one, you must make yourself a personal copy of that block before writing it, so you don’t disturb someone else’s turf. Here’s a simple example of reference counting and copy-on-write: //: C12:ReferenceCounting.cpp
// 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;
} ///:~
The class Dog is the object pointed to by a DogHouse. It contains a reference count and functions to control and read the reference count. There’s a copy-constructor so you can make a new Dog from an existing one. The attach( ) function increments the reference count of a Dog to indicate there’s another object using it. detach( ) decrements the reference count. If the reference count goes to zero, then no one is using it anymore, so the member function destroys its own object by saying delete this. Before you make any modifications (such as renaming a Dog), you should ensure that you aren’t changing a Dog that some other object is using. You do this by calling DogHouse::unalias( ), which in turn calls Dog::unalias( ). The latter function will return the existing Dog pointer if the reference count is one (meaning no one else is pointing to that Dog), but will duplicate the Dog if the reference count is more than one. The copy-constructor, instead of creating its own memory, assigns Dog to the Dog of the source object. Then, because there’s now an additional object using that block of memory, it increments the reference count by calling Dog::attach( ). The operator= deals with an object that has already been created on the left side of the =, so it must first clean that up by calling detach( ) for that Dog, which will destroy the old Dog if no one else is using it. Then operator= repeats the behavior of the copy-constructor. Notice that it first checks to detect whether you’re assigning the same object to itself. The destructor calls detach( ) to conditionally destroy the Dog. To implement copy-on-write, you must control all the actions that write to your block of memory. For example, the renameDog( ) member function allows you to change the values in the block of memory. But first, it uses unalias( ) to prevent the modification of an aliased Dog (a Dog with more than one DogHouse object pointing to it). And if you need to produce a pointer to a Dog from within a DogHouse, you unalias( ) that pointer first. main( ) tests the various functions that must work correctly to implement reference counting: the constructor, copy-constructor, operator=, and destructor. It also tests the copy-on-write by calling renameDog( ). Here’s the output (after a little reformatting): Creating Dog: [Fido], rc = 1
Created DogHouse: [FidoHouse]
contains [Fido], rc = 1
Creating Dog: [Spot], rc = 1
Created DogHouse: [SpotHouse]
contains [Spot], rc = 1
Entering copy-construction
Attached Dog: [Fido], rc = 2
DogHouse copy-constructor:
[copy-constructed FidoHouse]
contains [Fido], rc = 2
After copy-constructing bobs
fidos:[FidoHouse] contains [Fido], rc = 2
spots:[SpotHouse] contains [Spot], rc = 1
bobs:[copy-constructed FidoHouse]
contains [Fido], rc = 2
Entering spots = fidos
Detaching Dog: [Spot], rc = 1
Deleting Dog: [Spot], rc = 0
Attached Dog: [Fido], rc = 3
DogHouse operator= : [FidoHouse assigned]
contains [Fido], rc = 3
After spots = fidos
spots:[FidoHouse assigned] contains [Fido],rc = 3
Entering self-assignment
DogHouse operator= : [copy-constructed FidoHouse]
contains [Fido], rc = 3
After self-assignment
bobs:[copy-constructed FidoHouse]
contains [Fido], rc = 3
Entering rename("Bob")
After rename("Bob")
DogHouse destructor: [copy-constructed FidoHouse]
contains [Fido], rc = 3
Detaching Dog: [Fido], rc = 3
DogHouse destructor: [FidoHouse assigned]
contains [Fido], rc = 2
Detaching Dog: [Fido], rc = 2
DogHouse destructor: [FidoHouse]
contains [Fido], rc = 1
Detaching Dog: [Fido], rc = 1
Deleting Dog: [Fido], rc = 0
By studying the output, tracing through the source code, and experimenting with the program, you’ll deepen your understanding of these techniques. Automatic operator= creation
Because assigning an object to another object of the same type is an activity most people expect to be possible, the compiler will automatically create a type::operator=(type) if you don’t make one. The behavior of this operator mimics that of the automatically created copy-constructor; if the class contains objects (or is inherited from another class), the operator= for those objects is called recursively. This is called memberwise assignment. For example, //: C12:AutomaticOperatorEquals.cpp
#include <iostream>
using namespace std;
class Cargo {
public:
Cargo& operator=(const Cargo&) {
cout << "inside Cargo::operator=()" << endl;
return *this;
}
};
class Truck {
Cargo b;
};
int main() {
Truck a, b;
a = b; // Prints: "inside Cargo::operator=()"
} ///:~
The automatically generated operator= for Truck calls Cargo::operator=. In general, you don’t want to let the compiler do this for you. With classes of any sophistication (especially if they contain pointers!) you want to explicitly create an operator=. If you really don’t want people to perform assignment, declare operator= as a private function. (You don’t need to define it unless you’re using it inside the class.)
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C++ Programming 
