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

Google Earth 7.1.7.2602 - View and contr...
Google Earth gives you a wealth of imagery and geographic information. Explore destinations like Maui and Paris, or browse content from Wikipedia, National Geographic, and more. Google Earth combines... Read more
ClamXav 2.10 - Virus checker based on Cl...
ClamXav is a popular virus checker for OS X. I have been working on ClamXav for more than 10 years now, and over those years, I have invested a huge amount of my own time and energy into bringing... Read more
Tweetbot 2.4.4 - Popular Twitter client.
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Sierra Cache Cleaner 11.0.1 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Things 2.8.8 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
Remotix 4.1 - Access all your computers...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
Airfoil 5.1.2 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
Firefox 49.0.1 - Fast, safe Web browser.
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
Default Folder X 5.0.7 - 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 on... Read more
Safari Technology Preview 10.1 - The new...
Safari Technology Preview contains the most recent additions and improvements to WebKit and the latest advances in Safari web technologies. And once installed, you will receive notifications of... Read more

4 games like Burly Men at Sea to inspire...
Burly Men at Sea is out today and it looks a treat. It tells the tale of three Scandinavian fishermen who leave the humdrum of their daily lives to go exploring. It’s a beautiful folksy story that unfurls as you interact with the environment... | Read more »
3 reasons you need to play Kingdom: New...
Developed by a tag team of indie developers - Thomas "Noio" van den Berg and Marco "Licorice" Bancale - Kingdom is a vibrant medieval fantasy adventure that casts players as a king or queen who must expand their empire by exploring the vasts lands... | Read more »
JoyCity have launched a brand new King o...
Great news for all of you Game of Dice fans out there - JoyCity have just released a brand new limited edition pack with a really cool twist. The premise of Game of Dice is fairly straightforward, asking you to roll dice to navigate your way around... | Read more »
Burly Men at Sea (Games)
Burly Men at Sea 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burly Men at Sea is a folktale about a trio of large, bearded fishermen who step away from the ordinary to seek adventure. | Read more »
3 tips for catching the gnarliest waves...
Like a wave breaking on the shore, Tidal Rider swept its way onto the App Store charts this week settling firmly in the top 10. It’s a one-touch high score-chaser in which you pull surfing stunts while dodging seagulls and collecting coins. The... | Read more »
The beginner's guide to destroying...
Age of Heroes: Conquest is 5th Planet Games’ all new turn-based multiplayer RPG, full of fantasy exploration, guild building, and treasure hunting. It’s pretty user-friendly as far as these games go, but when you really get down to it, you’ll find... | Read more »
Infinite Tanks (Games)
Infinite Tanks 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: | Read more »
Agatha Christie - The ABC Murders (FULL)...
Agatha Christie - The ABC Murders (FULL) 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Agatha Christie: The ABC Murders Your weapon is your knowledge. Your wits will be put to the ultimate... | Read more »
HeadlessD (Games)
HeadlessD 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: HeadlessD is hand-painted dungeon crawler with intuitive touch controls and NO in-app purchases. | Read more »
Leaf for Twitter (Social Networking)
Leaf for Twitter 1.0.1 Device: iOS iPhone Category: Social Networking Price: $4.99, Version: 1.0.1 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more
MacBook Airs on sale for up to $100 off MSRP
B&H Photo has 13″ and 11″ MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799 $100 MSRP - 11″ 1.6GHz/256GB... Read more
Apple refurbished 12-inch 128GB iPad Pros ava...
Apple has Certified Refurbished 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 12″ iPad... Read more
Phone2Action Unveils New Voter Turnout Techno...
Phone2Action, a leading digital advocacy platform, today launched its Tech to Vote Civic Action Center digital advocacy and communications platform on National Voter Registration Day September 27.... Read more
Apple & Deloitte Team Up to Help Business...
Apple and international professional services firm Deloitte have announced a partnership to help companies quickly and easily transform their workflow dynamics by maximizing the power, ease-of-use,... Read more
Chop Commute – See Traffic and Drive Times on...
Shrewsbury, Massachusetts based Indie developer, InchWest has released Chop Commute 1.61, a Mac app that takes the guesswork out of daily commute by showing real-time traffic and drive times right on... Read more
12-inch 32GB WiFi iPad Pros on sale for $50 o...
B&H Photo has 12″ 32GB WiFi Apple iPad Pros on sale for $50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off MSRP -... Read more
Recent price drops on refurbished iPad minis...
Apple recently dropped prices on several Certified Refurbished iPad mini 4s and 2s as well as iPad Air 2s. An Apple one-year warranty is included with each model, and shipping is free: - 16GB iPad... Read more
Apple refurbished Mac minis available startin...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... Read more
13-inch 2.5GHz MacBook Pro available for $928...
Overstock has the 13″ 2.5GHz MacBook Pro available for $927.99 including free shipping. Their price is $171 off MSRP. Read more

Jobs Board

Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
*Apple* Wireless Lead - T-ROC - The Retail O...
…wealth of knowledge in wireless sales and activations to the Beautiful and NEW APPLE Experience store within MACYS.. THIS role, APPLE Wireless Lead, isbrandnewas Read more
Lead *Apple* Advocate - T-ROC - The Retail...
…Company, is proud of its unprecedented relationship with our partner and client, APPLE ,in bringing amazing" APPLE ADVOCATES"to "non" Apple store locations. Read more
*Apple* Advocate - T-ROC - The Retail Outsou...
…Company, is proud of its unprecedented relationship with our partner and client, APPLE ,in bringing amazing" APPLE ADVOCATES"to "non" Apple store locations. Read more
Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 51829230 Detroit, Michigan, United States Posted: Sep. 19, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC is an Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.