Why to use garbage collector?

Question

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Garbage Collection in C++ — why?

Hi, I read few articles about Garbage Collectors, and still there is one thing I just don´t understand - why use garbage collection?

I will try to explain my thoughts:

Garbage Collector should release dynamically allocated memory back to system in case there is no need for it, right? So, if you write program in language C, you know wheather you need some piece of memory, so if don´t, you can simply destroy it.

So, why to use GC, when all you need to do is actually just be wise with memory allocation/deallocation? Or am I missing something? Thanks.

Answer 1

To be more productive. In other words, the programmer can focus on writing the bits that is unique for his particular problem.

Answer 2

To avoid errors. No matter how careful you are about deallocating memory, either you will eventually make a mistake or you will eventually code a program that requires a complex memory reference pattern which will make the likelihood of error much greater.

Any possibility that exists for given enough time will become a reality, eventually you will leak memory with manual methods, unless extra effort is specifically put into monitoring memory consumption. This extra effort steals time from coding toward the primary purpose of the program, which probably isn't to manage memory.

In addition, even if your program doesn't leak memory, garbage collection often tends to handle memory more efficiently than many non-garbage collection methods. Most people don't new blocks of objects to avoid multiple new calls, nor would they revisit and clean up the cache of unused newed objects afterwards. Most manual garbage collection methods concentrate on freeing memory at block boundaries, which might let garbage linger a bit longer than it needed.

Each added benefit and feature you pile onto manual garbage collection takes you one step closer to automatic garbage collection. Using no utilities to collect garbage beyond the manual calls to free it will not scale easily. Either you will spend a lot of your time checking memory allocation / reclimation, or you will not spend enough to avoid a memory leak.

Either way, automatic garbage collection solves this problem for you, allowing you to get back to the main point of your program.

Answer 3

Because we are not living in the early 80s anymore. It's a waste of a developers time and it's simply annoying to care about lowest level tasks when you are about to create an amazing application.

Answer 4

Because we are not wise enough.

Answer 5

When I write programs, I like to concentrate on my problem domain and not on the minutiae of unrelated implementation details. For example, if I'm writing, say, a web server, my problem domain is network connections, protocols, data transmission/reception, and so on, not memory allocation and deallocation. If I'm writing a video game, my problem domain is graphical performance and perhaps AI and again not memory allocation and deallocation.

Any time I spend working on things that are not part of my problem domain is time wasted that could be spent on, you know, my problem domain. By concentrating on low-level details, in other words, the quality of my actual work—the problem I'm actually trying to solve—suffers.

Further, your "all you need to do is actually just be wise with memory allocation/deallocation" bit only serves to highlight two possibilities (that I can think of, at any rate):

1. You are very inexperienced.
2. You have been – likely subconsciously – crippling your designs to keep them within the restrictions of your simplistic assumptions about memory management.

Memory management in real-world software is a decidedly non-trivial endeavour. Complex data structures typical of any sizeable piece of modern software lead to incredible obfuscation in terms of determining the "liveness" and "ownership" of any given piece of dynamically-allocated memory. This is rendered even more complex (potentially by orders of magnitude) with the introduction of threading or, worse, multi-processing (both symmetric and otherwise) with any form of shared state.

It is no accident that the most common bugs in software written in unmanaged memory environments are related to poorly-managed dynamically allocated memory.

So why use garbage collection? Because you're not as smart as you think you are when dealing with the vagaries of dynamically allocated memory. Really, you're not. No matter how smart you think you are. If you recognize that you're not that smart, you cripple your designs to keep your memory management simple enough to understand. If you arrogantly believe, however, that you can deal with anything, you just screw your users who have to deal with your crappy, crash-prone and/or memory-eating software.

Answer 6

Consider a case where a particular pointer is used by two separate sub systems. One sub system may be done with the variable and the programmer may think, "I'm done with this, I'll just go ahead and free it", completely unaware that another sub system still needs access to it. Or another pitfall, the developer thinks, "I'm not sure if there is another sub system which may need this" (even if there is not) leading to memory leaks. This kind of situation comes up a lot in complex systems.

Answer 7

I agree with mouviciel's comment. But garbage collectors do allow for quicker development because the developer no longer has to worry about memory leaks, allowing them to focus on other aspects of their program.

But do note, that if you are programming on a language that has garbage collection, it is very wise to be aware of that fact. Its almost a must (IMO) to understand how it works, and what it is doing in the background.

Answer 8

It is an anti-dumb-programmer mechanism. And trust me, when code becomes very complex, when thinking in terms of dynamically allocated memory, we are all equally dumb.

In my short experience as a programmer I've spent (cumulated) days trying to figure out why valgrind (or other similar tools) is reporting memory leaks, when everything was so "wisely coded".

Answer 9

These days, most people who use a garbage collector are doing so inside a managed environment (like the Java Virtual Machine or the .NET Common Language Runtime). These managed environments add an additional wrinkle: they constrain the ability to take pointers to things. In the CLR for example, there is a notion of a pointer (which you can use through the managed IntPtr or the unmanaged unsafe code block), but there are limited conditions where you're allowed to use them. In most cases, you have to "pin" the corresponding objects in memory so that the GC doesn't move them while you're working with their pointers.

Why does this matter? Because, as it turns out, a managed allocator that is allowed to update pointers and move objects around in memory can be much more efficient than a malloc-style allocator. You can do cool things like generational garbage collection, which makes heap allocations as fast as stack allocations, you can profile the memory behavior of your application much more easily, and, oh yeah, you can also easily detect unreferenced objects and free them automatically.

So it's not only a matter of increased programmer productivity (although if you ask anyone who works in a managed language, they'll attest to the increased productivity it gives them), it's also a matter of enabling entirely new programming technologies.

Finally, garbage collection becomes truly necessary when working with functional programming languages (or programming in functional styles). In fact, the very first garbage collector was invented by McCarthy in 1959 as part of the development of the Lisp language. The reason is twofold: first, functional programming encourages immutable data structures, which are easier to collect, and second, in pure functional programming there is no allocation function; memory always gets allocated as "stack" (function locals) and then moves to a "heap" if it is captured by a closure. (This is a gross oversimplification but serves to illustrate the point.)

So... if you're programming in an imperative style, and you're "wise enough" to do the Right Thing will all your memory allocations, you don't need garbage collection. But if you want to change your programming style to take advantage of the newest advances in programming technology, you'll probably be interested in using a garbage collector.

Answer 10

When working in complex projects with multiple calls to libraries and external code that you did not write, it becomes very difficult to keep track of the objects that you need to free and the objects freed by external libs and other places in your code.

There are lots of tools that now exist which make the task of tracking down memory leaks easier, but they tend to be insidious bugs that only become noticeable after the system has been running for hours or days.

However, I do agree with your sentiment. If I have control over the code base, I prefer to write in something where I am in charge (like c). But if I have to work with external forces, something with a decent garbage collector is much more appealing.

Answer 11

So, if you write program in language C, you know wheather you need some piece of memory, so if don´t, you can simply destroy it.

That's the theory, at least. Problem is that it can complicate code greatly. For example, this:

for (x in ComputeBigList()) ...

becomes this

var xs = ComputeBigList();

try {
   for(x in xs) ...
} finally {
   FreeMemory(xs);
}

The lack of a garbage collector required us to name the result of ComputeBigList, store it in a variable and then add a delete statement wrapped in a finally, just to be sure it actually got deleted.

This is where C++ fans should be pointing out that C++'s guaranteed destructor calls can make this much easier. That said, you then have the overhead and additional code associated with reference counting, etc, assuming you want your objects to be able to escape the dynamic extent in which they were created. (ie: I allocate an object and then return it.)

The other thing GC does that's useful is control how you use your memory. A relocating GC lets you arrange objects so that they can be more efficiently accessed. GC in general gives your runtime a bit more flexibility about when you pay the price of reclaiming memory. (Explicit frees and refcount updates always have to be immediate.)

Answer 12

You do not need garbage collection if you do not produce garbage in the first place.

One way to avoid garbage is to not use dynamic memory allocation at all. Most embedded programs do not use dynamic memory allocation. Even when dynamic memory allocation is used (even in many PC programs) there is often no real reason to use it. (Just because dynamic memory allocation is possible does not mean it should be used everywhere.)

Another way to avoid garbage collection is to use language that does not separate reference from contents. In that case, actual memory leak is not even possible. (But of course it is still possible to use too much memory.) IMHO, high level languages should not mess with "pointers" (address variables) at all.

Answer 13

Releasing memory that is not needed anymore is an ideal goal, but it is not possible to do it automatically in all generality. Even in the absence of external input (which may affect whether some piece of data will be needed or not), deciding, given the complete state of the memory and the complete code, whether some piece of memory will be needed is equivalent to the halting problem, which is impossible to solve for a computer.

Needless to say, the same problem also exceeds the capacities of the average programmer brain quite fast, as the application size grows. Perfectly correct memory management can be achieved, in practice, only in two situations:

1. the problem is simple (e.g. short-lived command-line application) and the programmer disciplined enough;
2. the programmer is Donald Knuth.

In all other cases, we have to use approximations. A garbage collector relies on the following approximation: it detects unreachable blocks of memory. It cannot tell whether a reachable block will be used or not, but an unreachable block will not be used (because using implies reaching). Another common approximation (used by many programmers who feel they are wise enough) is to simply assume that they thought of every block, and then pray for the best (a variant being: educate your users into believing that memory leaks are a feature, and that a reboot every now and then is normal).

Answer 14

When you don't have to do real time application (you can't be sure of when the garbage collector will do his job event if you force him) or when you don't mind to fully control your memory, you can develop the head free and almost be sure to not make a memory leak.

Answer 15

Garbage collections can be more efficient.

To allocate memory, malloc needs to fiddle around to find a large enough contiguous span of memory. With a compacting garbage collector, allocating memory is bumping a pointer (or close to it)

In C++, you can safely and cleanly deal with memory in many situations without a garbage collector by using smart pointers and strictly adhering to conventions. But (1) this does not work in all situations, even with shared_ptr and weak_ptr, and (2) reference counting requires coordination across threads, which has a performance penalty.

Usability is more important concern, but garbage collection is, at times, faster than deterministically freeing memory.

Answer 16

you know wheather you need some piece of memory, so if don´t, you can simply destroy it.

You could use a similar argument to justify just about any labour saving device. Why write mathematical expressions when you can just produce assembly language? Why use readable characters when you can use binary?

The reason is simple. I work with programmers who are some of the best in their field. I can say without fear of exaggeration that some of them have written the book on their field. And yet these people are programming in C++ and make mistakes with memory management. When they make these mistakes, they are particularly difficult to find and correct. Why have amazing people whose talents could be directed elsewhere waste their time doing something a machine could do better?

(And yes, there are good answers to this question. For example when every byte of memory in your counts and so you can't afford to have any garbage at any time. But that is not the case in general.)

Answer 17

So, why to use GC, when all you need to do is actually just be wise with memory allocation/deallocation?

The problem is that it gets exceptionally hard to be sufficiently wise. Fail in the wisdom stakes and you get a memory leak or a crash. Here's the quick potted guide to computer-applied wisdom in automated memory management.

If you've got a simple program (the zero'th level of complexity), you can just use stack-based allocation to handle everything. It's very easy to get memory allocation right this way, but it's also a very restricted model of computation (and you also run into problems with stack space). So you start using the heap; that's where the “fun” begins.

The first level of complexity is where you've got pointers whose lifetime is bound to a stack frame. Again, that's fairly simple to do and forms the basis for much C++ programming.

The second level of complexity is where you've got reference counting. This is the basis for a C++ smart pointers, and is quite good at handling just about everything up to a forest of directed acyclic graphs. You can achieve a lot with this, and it permits some models of computation that work rather nicely with functional programming and parallel programming too.

Beyond that is the third level, garbage collection. This can handle arbitrary graphs of memory structures, but with a cost of being more memory hungry (since you don't in general try to deallocate quite as soon). One of the main costs is that the amount of memory allocated tends to be larger, due to the fact that it's only after the point where you could have deleted the memory that it really becomes eligible for automated deletion, were you but smart enough to figure out the lifetimes.

Answer 18

You just need to be wise, that's right ;) However, if your design isn't right it could be easy to oversee something.

With Garbage Collection however you don't have to care about memory and can focus more on the rest of your program, thus possibly develop "faster"

Answer 19

You can do your own garbage collection, as you mentioned. Adding a garbage collector simply frees you from having to worry about it and from having to take the time to write and test your garbage collection code. If you are working on an existing codebase that contains memory leaks, it can be easier (and more effective) to use an automatic garbage collector than to try to learn all of the existing code in enough detail to find and fix the problems.

That being said, I'm not a fan of adding automatic garbage collection facilities to languages that don't have it built in. If the language was designed assuming that the developer would be thoughtful about memory allocation and de-allocation, then (IMHO) it does the developer a disservice to remove this responsibility from them. Not being able to control precisely when and how memory is freed can lead to inconsistent behavior. Thinking about and defining the full lifetime of dynamically-allocating memory is an important part of planning your code. No automated system is a true substitute for careful and accurate programming (that applies to far more than just garbage collectors).

Answer 20

Without a garbage collector, anytime you allocate something dynamically, you have to keep track of when you no longer need it, and destroy it only after you no longer need it. This can be difficult, especially when/if a number of different parts of the program all have pointers to one object, and no one of them knows what other code might be using it.

That's the theory anyway. In reality, I have to admit that it hasn't worked out that way for me either. In particular, when (most) people are aware that their code will be using a garbage collector, they tend to dismiss memory management as not being a problem or even an issue to consider at all. As a result, they can jump in and start coding more quickly. For small problems that they understand quite well before starting, this can be a significant win -- but for larger problems it appears (at least to me) that the tendency is toward jumping in and starting to write code before they really understand the problem.

In my experience, lack of a garbage collector makes the developer(s) think a bit more about lifetime issues up-front. In the process, they're motivated to find simple solutions to object lifetime issues -- and they usually do exactly that. In the process, they've typically simplified the code in general (not just the memory management) to the point that it's much simpler, cleaner, and more understandable.

In a way, it reminds me a lot of the old parable of two programmers. At the end of a project, the managers who look at code that used garbage collection think it's a really good thing they used garbage collection. The problem is clearly even more complex than they realized, and given the complexity of the code and the lifetime issues, there's no way anybody could keep track of them by hand and produce code that was even close to leak-free.

At the end of doing the same thing without garbage collection, the reaction is rather the opposite. They realize that the problem (in general) is really a lot simpler than they had realized. Object lifetime issues aren't really nearly as complex as they'd expected, and producing leak-free code wasn't particularly challenging at all.

Answer 21

You can need release Interop resources as soon as posible (locked file). Gc.Collect can ensure COM Objects are released (if not referenced).

If you do a PrintPreview, this takes 2 Gdi handles for each Page (image + metafile). These resources are´nt released by PrintDocument or PrintControler, are waiting to GC.

I tested on a interactive program to use Gc.Collect when user return to main menu. With this operation, the reported memory for Task Manager is about 50%.

I think this is´nt important, but code a Gc.Collect when you know that a lot of memory is not referenced is a simple option.

Answer 22

memory management is implementation issue, that doesn't connected to the goal of program.

by goal of the program i mean things like business logic.

when you working on implementation issues - you throws your time and efforts on things that doesn't help you to finish the program.