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
$98.77
Apple Inc.
-0.26
MSFT
$44.00
Microsoft Corpora
+0.03
GOOG
$589.11
Google Inc.
-1.49

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Yosemite Wallpaper 1.0 - Desktop im...
OS X Yosemite Wallpaper is the gorgeous new background image for Apple's upcoming OS X 10.10 Yosemite. This wallpaper is available for all screen resolutions with a source file that measures 5,418... Read more
Acorn 4.4 - Bitmap image editor. (Demo)
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Bartender 1.2.20 - Organize your menu ba...
Bartender lets you organize your menu bar apps. Features: Lets you tidy your menu bar apps how you want. See your menu bar apps when you want. Hide the apps you need to run, but do not need to... Read more
TotalFinder 1.6.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
Vienna 3.0.0 RC 2 :be5265e: - RSS and At...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
VLC Media Player 2.1.5 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Default Folder X 4.6.7 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more
TinkerTool 5.3 - Expanded preference set...
TinkerTool is an application that gives you access to additional preference settings Apple has built into Mac OS X. This allows to activate hidden features in the operating system and in some of the... Read more
Audio Hijack Pro 2.11.0 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Intermission 1.1.1 - Pause and rewind li...
Intermission allows you to pause and rewind live audio from any application on your Mac. Intermission will buffer up to 3 hours of audio, allowing users to skip through any assortment of audio... Read more

Latest Forum Discussions

See All

Puzzix Review
Puzzix Review By Jennifer Allen on July 29th, 2014 Our Rating: :: NICE IDEAUniversal App - Designed for iPhone and iPad A little like Tetris, Puzzix is all about piecing together blocks and watching them vanish. It could do with... | Read more »
Cannonball eMail is Now Live – Works Wit...
Cannonball eMail is Now Live – Works With Gmail, Yahoo, Outlook, Hotmail, and AOL Posted by Jessica Fisher on July 29th, 2014 [ permalink ] | Read more »
To The End Review
To The End Review By Lee Hamlet on July 29th, 2014 Our Rating: :: A VICIOUS CYCLEUniversal App - Designed for iPhone and iPad To The End will test players’ patience, timing, and dedication as they try to navigate all 13 levels in... | Read more »
Kairobotica (Games)
Kairobotica 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: In a galaxy not so far away, miscreants and monsters are wreaking havoc, and it's up to everyone's favorite mechanical mascot... | Read more »
Traps n’ Gemstones Review
Traps n’ Gemstones Review By Campbell Bird on July 28th, 2014 Our Rating: :: CASTLEVANIA JONESUniversal App - Designed for iPhone and iPad Fight mummies, dig tunnels, and ride a runaway minecart to discover ancient secrets in this... | Read more »
The Phantom PI Mission Apparition Review
The Phantom PI Mission Apparition Review By Jordan Minor on July 28th, 2014 Our Rating: :: GHOSTS BUSTEDUniversal App - Designed for iPhone and iPad The Phantom PI is an exceedingly clever and well-crafted adventure game.   | Read more »
More Stubies Are Coming Your Way in a Ne...
More Stubies Are Coming Your Way in a New Update Posted by Jessica Fisher on July 28th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
The Great Prank War Review
The Great Prank War Review By Nadia Oxford on July 28th, 2014 Our Rating: :: PRANKING IS SERIOUS BUSINESSUniversal App - Designed for iPhone and iPad Though short, The Great Prank War offers an interesting and fun mix of action and... | Read more »
Marvel Contest of Champions Announced at...
Marvel Contest of Champions Announced at Comic-Con Posted by Jennifer Allen on July 28th, 2014 [ permalink ] Announced over the weekend at San Diego Comic-Con was the fairly exciting looking Marvel Contest of Champions. | Read more »
Teenage Mutant Ninja Turtles Review
Teenage Mutant Ninja Turtles Review By Jennifer Allen on July 28th, 2014 Our Rating: :: DULL SWIPINGUniversal App - Designed for iPhone and iPad The pizza power is weak when it comes to this Teenage Mutant Ninja Turtles game.   | Read more »

Price Scanner via MacPrices.net

Apple updates MacBook Pros with slightly fast...
Apple updated 13″ and 15″ Retina MacBook Pros today with slightly faster Haswell processors. 13″ models now ship with 8GB of RAM standard, while 15″ MacBook Pros ship with 16GB across the board. Most... Read more
Apple drops price on 13″ 2.5GHz MacBook Pro b...
The Apple Store has dropped their price for the 13″ 2.5GHz MacBook Pro by $100 to $1099 including free shipping. Read more
Apple drops prices on refurbished 2013 MacBoo...
The Apple Store has dropped prices on Apple Certified Refurbished 13″ and 15″ 2013 MacBook Pros, with model now available starting at $929. Apple’s one-year warranty is standard, and shipping is free... Read more
iOS 8 and OS X 10.10 To Support DuckDuckGo As...
Writing for Quartz, Dan Frommer reports that Apple’s forthcoming iOS 8 and OS X 10.10 operating systems version updates will allow users to select DuckDuckGo as their default search engine. He notes... Read more
U.K. Hospital Using iPods and iPads To Record...
British news journal GazetteLive’s. Ian McNeal notes that the old “an apple a day keeps the doctor away” proverb is being turned on its head at http://southtees.nhs.uk/hospitals/james-cook/ James... Read more
13-inch 2.5GHz MacBook Pro on sale for $1099,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $1099.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $100 off MSRP. Price is... Read more
Roundup of Apple refurbished MacBook Pros, th...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available for up to $400 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. Their prices... Read more
Record Mac Shipments In Q2/14 Confound Analys...
A Seeking Alpha Trefis commentary notes that Apple’s fiscal Q3 2014 results released July 22, beat market predictions on earnings, although revenues were slightly lower than anticipated. Apple’s Mac’... Read more
Intel To Launch Core M Silicon For Use In Not...
Digitimes’ Monica Chen and Joseph Tsai, report that Intel will launch 14nm-based Core M series processors specifically for use in fanless notebook/tablet 2-in-1 models in Q4 2014, with many models to... Read more
Apple’s 2014 Back to School promotion: $100 g...
 Apple’s 2014 Back to School promotion includes a free $100 App Store Gift Card with the purchase of any new Mac (Mac mini excluded), or a $50 Gift Card with the purchase of an iPad or iPhone,... Read more

Jobs Board

Sr Software Lead Engineer, *Apple* Online S...
Sr Software Lead Engineer, Apple Online Store Publishing Systems Keywords: Company: Apple Job Code: E3PCAK8MgYYkw Location (City or ZIP): Santa Clara Status: Full Read more
*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
Sr. Product Leader, *Apple* Store Apps - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*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
*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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.