TweetFollow Us on Twitter

Deferred Garbage Collection

Volume Number: 13 (1997)
Issue Number: 12
Column Tag: C++ Workshop

Deferred Garbage Collection

by Marc White

Implementing a simple deferred C++ object garbage collection class

Self Deleting Objects

When developing object oriented software using C++ there are certain situations where you may need an object that can delete itself. For instance, you might have a window or dialog class that manages its own mouse clicks and key down events. But what does the dialog object do when it determines that the user has pressed the escape or delete key, or clicked the window's close box? To stay true to the object oriented design, the window should be able to do whatever processing is needed when it determines that it is being closed and then free up the memory allocated for the object by itself.

Delete This

While the C++ syntax delete this is a legal call, it can cause serious problems if the object that makes the call is referenced in any way after the call is made. This could possibly happen while the stack is unwinding after a mouse click or key down event.

The TGarbageCollector Class

The TGarbageCollector class is a simple, drop in utility class that allows any C++ object to be safely deleted at a later time.

Listing 1: The TGarbageCollector Class

The TGarbageCollector class is a non-instance class with two public static methods: Add, and Empty, and one private static member variable: fTrashCan which is pointer to a TGarbage object.

class TGarbageCollector {
friend class TGarbage;
public:
  static void Add      ( void *trash );
  static void Empty    ( void );
  
private:
  static TGarbage      *fTrashCan;
};

TGarbageCollector::Add

The Add method of the TGarbageCollector class simply instantiates a new TGarbage object passing it a pointer to the C++ object to be deleted. Note that the trash pointer is a void pointer.

void TGarbageCollector::Add( void *trash )
{
  // create a new TGarbage object
  new TGarbage( trash );
}

TGarbageCollector::Empty

The Empty method of the TGarbageCollector class deletes all of the TGarbage objects pointed to by the fTrashCan variable. This method should be called periodically at idle time in the application's main event loop. The TGarbage object's destructor maintains the linked list.

void TGarbageCollector::Empty( void )
{
  // delete all of the items in the trash can
  while( TGarbageCollector::fTrashCan )
    delete( TGarbageCollector::fTrashCan );
}

I developed the TGarbageCollector class as a part of an application framework I was designing. The easiest way to design the garbage collection class would have been to have it delete only objects derived from a specific class, perhaps a TTrashableObject class. But, I wanted the garbage collector to be able to delete any C++ object, and I didn't want to have every class in the framework be derived from a single base class.

The TGarbage Class

This is where the TGarbage class comes in. Because the TGarbage class accepts a void pointer as a pointer to the object it will delete, any pointer can be passed into the garbage collection class to be deleted. This means that it is up to the developer (not the compiler) to make sure that the pointer passed in is a pointer to a valid C++ object.

There is one more catch which I will describe in the What's the Catch section, but for now let's take a look at the TGarbage class.

Listing 2: The TGarbage Class

The TGarbage class is a simple self-linking singly linked list class which accepts a void pointer through its constructor. It stores a pointer to the object to delete in the fTrash variable and a pointer to the next TGarbage object in the list in its fNext variable.

class TGarbage {
public:
            TGarbage    ( void *trash );
  virtual    ~TGarbage  ( void );
private:
  void        *fTrash;
  TGarbage    *fNext;
};

TGarbage::TGarbage

The TGarbage constructor stores a pointer to object to delete in its fTrash variable. Next it links itself into the linked list pointed to by the TGarbageCollector's static fTrashCan variable. It can access this private item since the TGarbageCollector has the declared the TGarbage class as a friend.

TGarbage::TGarbage( void *trash ) : fTrash( trash )
{
  // store a pointer to the next item in the chain
  if( TGarbageCollector::fTrashCan )
    this->fNext = TGarbageCollector::fTrashCan;
  else
    this->fNext = nil;
  
  // set this item as the first item in the trash can
  TGarbageCollector::fTrashCan = this;
}

TGarbage::~TGarbage

This is where the real magic happens. When the TGarbage object gets deleted we have no idea what type of object its fTrash variable points to, all that we know is that it is pointing to a C++ object. So just type cast it to a C++ class pointer (in this case a TGarbage pointer) and call the delete operator. The real object's destructor will get called and its memory will be deallocated. It's just that easy!

TGarbage::~TGarbage( void )
{
  // pull this object out of the linked list
  TGarbageCollector::fTrashCan = this->fNext;
  
  // delete the trash
  delete( (TGarbage *)this->fTrash );
}

Sample Usage

Let's take a look at the TGarbageCollector class in action. Code listing 3 shows the DoKeyDown method of a typical dialog class.

Listing 3: A Dialog Class Method

void FDialog::DoKeyDown( char theKey )
{
  switch( theKey ) {
    case kEscKey:
      // add this object to the trash
      TGarbageCollector::Add( this );
      // hide this dialog
      this->Hide();
      // do any other processing needed here before closing
      break;
    default:
      inherited::DoKeyDown( theKey );
  }
}

Notice that the dialog can add itself to the trash at any time once it determines that it needs to be deleted. The dialog object is still valid until the trash gets collected.

Next we'll take a look at the event loop method of a typical application class. This is where the TGarbageCollector's Empty method gets called and all of the objects added to the trash are deleted.

Listing 4: An Application Class Event Loop Method

void FApplication::EventLoop( void )
{
  EventRecord  theEvent;

  while( this->fQuit == false ) {
    if( WaitNextEvent( everyEvent, &theEvent, 30, nil ) ) {
      switch( theEvent.what ) {
        // handle all normal events here
        default:
          TGarbageCollector::Empty();
          break;
      }
    } else {
      TGarbageCollector::Empty();
    }
  }
}

What's The Catch?

So there has to be a catch, right? Well, of course there is. The catch is that any C++ object added to the trash must be structured in the same way as the object used to type cast the void pointer in the TGarbage object before calling its delete operator.

In simpler terms, any object added to the trash must have a virtual destructor, and its destructor must be the first virtual method of that class. This is because of the way a C++ object is structured from a class.

The TGarbage class typecasts the void pointer to the object it is going to delete into a pointer to a TGarbage object. This instructs the compiler to use the virtual table, or vtbl, of the TGarbage class to find the location of the object's destructor which it calls before deallocating the object's memory.

In the case of the TGarbage class, the destructor is the first virtual method of the class and therefore will be the first entry in the vtbl. As long as any object added to the trash can has its destructor as the first item in the vtbl, it will be properly called when the TGarbage class deletes the object.

What would happen if the object being trashed did not have a virtual destructor, or if the destructor was not the first virtual method? If the object's virtual destructor is not the first virtual method in the class, or it's destructor is not virtual, then before the memory for that object is deallocated the first virtual method of that class will be called. In the case of an object that does not have any virtual methods, the object will not even have a vtbl which means that some random memory location is going be executed as a method. Obviously, neither of these two scenarios are desirable, which is why it is very important that the developer structures the classes for trashable objects properly.

Multiple Inheritance

Can objects that use multiple inheritance be successfully deleted using the TGarbageCollector class? In short, yes, as long as they still adhere to the virtual-destructor-first method like the other classes.

However, there is an exception. Depending upon how the compiler implements multiple inheritance (I am using CodeWarrior for this example), as long as the first class specified in the inheritance chain has a virtual destructor as its first virtual method, the order of the virtual methods of the other superclasses does not matter.

Listing 5: Multiple Inheritance

// destructor is the first virtual method
class A {
public:
              A    ( void );
  virtual      ~A    ( void );
};

// destructor is the second virtual method
class B {
public:
              B    ( void );
  virtual void  Test  ( void );
  virtual      ~B    ( void );
};

// inherit from A first, then B
class C : public A, public B {
public:
              C    ( void );
  virtual      ~C    ( void );
};

// inherit from B first, then A 
class D : public B, public A {
public:
              D    ( void );
  virtual      ~D    ( void );
};

In code listing 5, class A follows the virtual-destructor-first method which would allow any object of class A to be deleted properly by the TGarbageCollector class. Class B, however, has its virtual destructor declared as the second virtual method in the class, and therefore, would not be deleted properly by the TGarbageCollector class. The B object's Test method would actually get called when the TGarbage class deleted it.

Since class C inherits from class A first and class B second, class C objects can successfully be deleted using this method. When the TGarbage class calls the C object's delete operator, the class C destructor will be called first, the class B destructor second, and the A destructor third.

Class D, however, inherits from the B class first, which means that it will behave exactly like a B object would when being deleted by the TGarbageCollector, the inherited B object's Test method will get called.

So, even though there is an exception to the virtual-destructor-first rule in the case of multiple inheritance, it is much safer to make sure that any object that might be deleted by the TGarbageCollector have its virtual destructor declared before any other virtual methods.

Summary

The TGarbageCollector class provides a simple means of adding deferred object deletion to any C++ application. However, the developer must make certain that the classes of objects which will be deleted in this manner are structured according to the virtual-destructor-first rule.


Marc White is a Macintosh programmer at US WEST Dex, Inc., where he develops client/server software used to paginate the US WEST phone directories. He can be reached at mwhite@eagle.mrg.uswest.com.

 
AAPL
$102.05
Apple Inc.
+2.29
MSFT
$44.80
Microsoft Corpora
+0.72
GOOG
$521.93
Google Inc.
+1.09

MacTech Search:
Community Search:

Software Updates via MacUpdate

RapidWeaver 6.0 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
NTFS 12.0.39 - Provides full read and wr...
Paragon NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and NTFS, providing full read and write access to... Read more
RestoreMeNot 2.0.3 - Disable window rest...
RestoreMeNot provides a simple way to disable the window restoration for individual applications so that you can fine-tune this behavior to suit your needs. Please note that RestoreMeNot is designed... Read more
Macgo Blu-ray Player 2.10.9.1750 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple iOS 8.1 - The latest version of Ap...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
TechTool Pro 7.0.5 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PDFKey Pro 4.0.2 - Edit and print passwo...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Yasu 2.9.1 - System maintenance app; per...
Yasu was originally created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks... Read more
Hazel 3.3 - Create rules for organizing...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a... Read more
Autopano Giga 3.7 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more

Latest Forum Discussions

See All

Inferno 2 Review
Inferno 2 Review By Andrew Fisher on October 21st, 2014 Our Rating: :: TWIN STICK GOODNESSUniversal App - Designed for iPhone and iPad With tight controls and awesome, stark visuals, Inferno 2 is loads of fun.   | Read more »
Clips Review
Clips Review By Jennifer Allen on October 21st, 2014 Our Rating: :: CONVENIENT PASTINGUniversal App - Designed for iPhone and iPad Making copying and pasting more powerful than usual, Clips is a great way to move stuff around.   | Read more »
MonSense Review
MonSense Review By Jennifer Allen on October 21st, 2014 Our Rating: :: ORGANIZED FINANCESiPhone App - Designed for the iPhone, compatible with the iPad Organize your finances with the quick and easy to use, MonSense.   | Read more »
This Week at 148Apps: October 13-17, 201...
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Angry Birds Transformers Review
Angry Birds Transformers Review By Jennifer Allen on October 20th, 2014 Our Rating: :: TRANSFORMED BIRDSUniversal App - Designed for iPhone and iPad Transformed in a way you wouldn’t expect, Angry Birds Transformers is a quite... | Read more »
GAMEVIL Announces the Upcoming Launch of...
GAMEVIL Announces the Upcoming Launch of Mark of the Dragon Posted by Jessica Fisher on October 20th, 2014 [ permalink ] Mark of the Dragon, by GAMEVIL, put | Read more »
Interview With the Angry Birds Transform...
Angry Birds Transformers recently transformed and rolled out worldwide. This run-and-gun title is a hit with young Transformers fans, but the ample references to classic Transformers fandom has also earned it a place in the hearts of long-time... | Read more »
Hail to the King: Deathbat Review
Hail to the King: Deathbat Review By Rob Thomas on October 20th, 2014 Our Rating: :: SO FAR AWAYUniversal App - Designed for iPhone and iPad Hail to the King: Deathbat may feel like “Coming Home” for Avenged Sevenfold’s faithful,... | Read more »
Find Free Food on Campus with Ypay
Find Free Food on Campus with Ypay Posted by Jessica Fisher on October 20th, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Strung Along Review
Strung Along Review By Jordan Minor on October 20th, 2014 Our Rating: :: GOT NO STRINGSUniversal App - Designed for iPhone and iPad A cool gimmick and a great art style keep Strung Along from completely falling apart.   | Read more »

Price Scanner via MacPrices.net

Select MacBook Airs $100 off MSRP, free shipp...
B&H Photo has 2014 a couple of MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for... Read more
13-inch 2.5GHz MacBook Pro on sale for $100 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Strong iPhone, Mac And App Store Sales Drive...
Apple on Monday announced financial results for its fiscal 2014 fourth quarter ended September 27, 2014. The Company posted quarterly revenue of $42.1 billion and quarterly net profit of $8.5 billion... Read more
Apple Posts How-To For OS X Recovery
OS X 10.7 Lion and later include OS X Recovery. This feature includes all of the tools you need to reinstall OS X, repair your disk, and even restore from a Time Machine backup. OS X Recovery... Read more
Mac OS X Versions (Builds) Supported By Vario...
Apple Support has posted a handy resource explaining which Mac OS X versions (builds) originally shipped with or are available for your computer via retail discs, downloads, or Software Update. Apple... Read more
Deals on 2011 13-inch MacBook Airs, from $649
Daily Steals has the Mid-2011 13″ 1.7GHz i5 MacBook Air (4GB/128GB) available for $699 with a 90 day warranty. The Mid-2011 13″ 1.7GHz i5 MacBook Air (4GB/128GB SSD) is available for $649 at Other... Read more
2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Updated iPad Prices
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers, including the new iPad Air 2 and the... Read more
Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.