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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Monolingual 1.6.4 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. If you use your computer in only one (human) language, you... Read more
CleanApp 5.0 - Application deinstaller a...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Fantastical 2.0 - Create calendar events...
Fantastical is the Mac calendar you'll actually enjoy using. Creating an event with Fantastical is quick, easy, and fun: Open Fantastical with a single click or keystroke Type in your event details... Read more
Cocktail 8.2 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Direct Mail 4.0.4 - Create and send grea...
Direct Mail is an easy-to-use, fully-featured email marketing app purpose-built for OS X. It lets you create and send great looking email campaigns. Start your newsletter by selecting from a gallery... Read more
jAlbum Pro 12.6 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.6 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Lyn 1.5.9 - Lightweight image browser an...
Lyn is a lightweight and fast image browser and viewer designed for photographers, graphic artists and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it... Read more
Sublime Text 3080 - Sophisticated text e...
Sublime Text is a sophisticated text editor for code, markup, and prose. You'll love the slick user interface, extraordinary features, and amazing performance. Goto Anything. Use Goto Anything to... Read more
WALTR 1.0.11 - Drag-and-drop any media f...
WALTR is designed to make it easy to upload and convert any music or video file to an iPad or iPhone format for native playback. It supports a huge variety of media file types, including MP3, MP4,... Read more

Bio Inc's New Expansion is Infectin...
Bio Inc., by DryGin Studios, is the real time strategy game where you infect a human body with the worst virus your evil brain can design. Recently, the game was updated to add a whole lot of new features. Now you can play the new “Lethal”... | Read more »
The Monocular Minion is Here! Despicable...
Despicable Me: Minion Rush, by Gameloft, is introducing a new runner to the mix in their latest update. Now you can play as Carl, the prankster minion. Carl has a few new abilities to play with, including running at a higher speed from the start.... | Read more »
Dungeon of Madness (Games)
Dungeon of Madness 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: Dungeon of Madness is an action game where you rotate tiles to create our own route. Help the hero by connecting the... | Read more »
Filters for iPhone (Photography)
Filters for iPhone 1.0 Device: iOS iPhone Category: Photography Price: $.99, Version: 1.0 (iTunes) Description: | Read more »
Jump'N'Shoot Attack (Games)
Jump'N'Shoot Attack 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: A mobile game for gamers! Join Louise Lightfoot, the legendary "Master of Jumping and Shooting", on her mission to save... | Read more »
Space Bounties Inc. (Games)
Space Bounties Inc. 1.4 Device: iOS Universal Category: Games Price: $1.99, Version: 1.4 (iTunes) Description: SuperGameDroid: 4/5 "Satisfying futuristic RPG combat, high replay value, and a heavy dose of nostalgia make Space... | Read more »
Gamebook: Pocket RPG (Games)
Gamebook: Pocket RPG 1.0.11 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.11 (iTunes) Description: Walk into the Land of Lanthir Lamath ruled by wicked skeletons and fight for your life in a thrilling adventure.... | Read more »
Kids Can Mix, Match, and Catch with Tata...
Tatadada MixMatch, by Tatadada Ltd, is a mobile version of the classic game of mix & match. The game uses brightly colored creatures to train your children's pattern matching skills and hand-eye coordination. It's aimed at children around age 5... | Read more »
The Trace: Murder Mystery Game (Games)
The Trace: Murder Mystery Game 1.2.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.2.0 (iTunes) Description: | Read more »
This Week at 148Apps: March 16-20, 2015
Spring Roars In At 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out above... | Read more »

Price Scanner via MacPrices.net

Logitech Says MX Master Is Its Most Advanced...
Logitech’s new MX Master Wireless Mouse incorporates the best of Logitech’s many computer mouse innovations into a striking hand-sculpted design. The company claims that the MX Master creates a new... Read more
Save up to $300 on a new Mac, $30 on an iPad,...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
Apple refurbished 2014 MacBook Airs available...
The Apple Store lowered prices on Apple Certified Refurbished 2014 MacBook Airs recently, with models now available starting at $679. An Apple one-year warranty is included with each MacBook, and... Read more
Mac Notebook Evolution; A Desktop Replacement...
More often than not right from the beginning, Apple’s Macs have tended to skew toward small. The original Macs were called “compacts,”, and notwithstanding a few exceptions like the honking Big Mac... Read more
13-inch 1.4GHz/128GB MacBook Air (Apple refur...
The Apple Store has Apple Certified Refurbished 2014 13″ 1.4GHz/128GB MacBook Airs available for $759 including free shipping plus Apple’s standard one-year warranty. Their price is $240 off original... Read more
YEP! Alternative Browser for iOS Now Supports...
Pfaeffikon, Switzerland based Power App AG has announced the release of an update to their Yep! Web Browser (v1.3.0) for iOS8 iPhone and iPad. Yep! hit the App Store shortly after the release of iOS... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $250 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $... Read more
Clearance 13-inch Retina MacBook Pros availab...
B&H Photo has leftover 2014 13″ Retina MacBook Pros on sale for up to $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1098... Read more
Clearance 2014 MacBook Airs on sale for up to...
B&H Photo has MacBook Airs on sale for up to $180 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $789.99 110 off original MSRP - 11″ 256GB... Read more
Apple refurbished Time Capsules available for...
The Apple Store has certified refurbished Time Capsules available for $100 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $199, $100... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - D...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Systems Engineer - Pre Sales, Educat...
…is responsible for proactively providing technical expertise to drive sales of Apple solutions into assigned accounts. The SE architects, validates, and assists in Read more
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.