Programmer Musings

Object Death

What does it mean for an object to die? In C++, there are several distinct and well-defined stages in the death of an object. Other languages do this a little differently, but the general concepts remain the same.

This is the basic chain of events for an item on the stack.

1. Object becomes inaccessible
2. Object destructor is called
3. Memory is freed

Once an object goes out of scope it begins the process of dying. The first step in that process is calling the object's destructor. (To simplify the discussion, we will ignore the destructors of any ancestor classes.) The destructor should undo anything done by the object's constructor. Finally, after all of the destruction of the object is completed, the system gets an opportunity to recover the memory taken by the object.

In some other languages, a garbage collection system handles recovering memory. Some systems guarantee destruction when the object leaves scope, even with automatic garbage collection. However, some of them focus so hard on memory recovery that they provide no guarantees about when, or even if, destruction of the object will occur.

Although many people pay a lot of attention to the memory recovery part of this process, it seems to be the least interesting part of the process to me. The destruction of the object often plays a vital role in the lifetime of the object. This destruction often involves releasing resources acquired by the object. Sometimes, memory is the only thing to be cleaned up, but many times other resources must be released. Some examples include

• closing a file
• releasing a semaphore or mutex
• closing a socket
• closing/releasing a database handle
• terminating a thread

These are all issues that we would like to take care of as soon as possible. Also, they result in some consequence if the cleanup step is forgotten or missed.

Anytime I have a resource that must be initialized or acquired and shutdown or released, I immediately think of a class that wraps that functionality in the constructor and destructor. This pattern is often known as resource acquisition is initialization. Following this pattern gives you an easy way to tell when the resource is yours. Your ownership of the resource corresponds to the lifetime of the object. You can't forget to clean up, it is done automatically by the destruction of the object. Most importantly, the resource is even cleaned up in the face of exceptions.

In the systems where destruction may be postponed indefinitely, this very useful concept of object death and the related concept of object lifetime is discarded.

The Forgotten OO Principle

When talking about Object Oriented Programming, there are several principles that are normally associated with the paradigm: polymorphism, inheritance, encapsulation, etc.

I feel that people tend to forget the first, most important principle of OO: object lifetime. One of the first things that struck me when I was learning OO programming in C++ over a decade ago, was something very simple. Constructors build objects and destructors clean them up. This seems obvious, but like many obvious concepts, it has subtleties that make it worth studying.

In an class with well-done constructors, you can rely on something very important. If the object is constructed it is valid. This means that you generally don't have to do a lot of grunt work to make sure the object is set up properly before you start using it. If you've only worked with well-done objects, this point may not be obvious. Those of us who programmed before OO got popular remember the redundant validation code that needed to go in a lot of places to make certain that our data structures were set up properly.

Since that time, I have seen many systems where the programmers forgot this basic guarantee. Every time this guarantee is violated in the class, all of the client programmers who use this class have a lot more work on their hands.

I'm talking about the kind of class where you must call an initialize method or a series of set methods on the object immediately after construction, otherwise you aren't guaranteed useful or reliable results. Among other things, these kinds of objects are very hard for new programmers to understand. After all, what is actually required to be set up before the object is valid? There's almost no way to tell, short of reading all of the source of the class and many of the places where it is used.

What tends to happen in these cases is the new client programmer copies code from somewhere else that works and tweaks it to do what he/she needs it to do. This form of voodoo programming is one of the things that OO was supposed to protect us from. Where this really begins to hurt is when a change must be made to the class to add some form of initialization, how are you going to fix all of the client code written with it. Granted, modern IDEs can make some of this a little easier, but the point is that I, as the client of the class, will need to change the usage of the object possibly many times if the class implementation changes.

That being said, it is still possible to do some forms of lazy initialization that save time at construction time. But, the guarantee must still apply for a good class. After construction, the object must be valid and usable. If it's not, you don't have an object, you have a mass of data and behavior.

The other end of the object's lifetime is handled by a destructor. When an object reaches the end of it's life, the destructor is called undoing any work done by the constructor. In the case of objects that hold resources, the destructor returns those resources to the system. Usually, the resource is memory. But, sometimes there are other resources, such as files, database handles, semaphores, mutexes, etc.

If the object is not properly destroyed, then the object may not be accessible, but it doesn't really die. Instead, it becomes kind of an undead object. It haunts the memory and resource space of the process until recovered by the death of the whole process. I know, it's a little corny. But, I kind of like the imagery.

This concept also explains one of the problems I have with some forms of garbage collection. Garbage collection tends to assume that the only thing associated with an object is memory. And, as long as the memory is returned before you need it again, it doesn't really matter when the object dies. This means that we will have many of these undead objects in the system at any time. They are not really alive, but not yet fully dead. In some cases, you are not even guaranteed that the destructor, or finalizer will be called. As a result, the client programmer has to do all of the end of object clean up explicitly. This once again encourages voodoo programming as we have to copy the shutdown code from usage to usage throughout the system.

So keep in mind the importance of the lifetime of your objects. This is a fundamental feature of object oriented programming that simplifies the use of your classes, and increases their usefulness.