Freestore management

Part of C++ FQA Lite. To see the original answers, follow the FAQ links.

This page is about one of the most hard, boring and dangerous things C++ forces you to do manually - killing your objects at the right time. One dirty job, that.

[16.1] Does delete p delete the pointer p, or the pointed-to-data *p?

FAQ: That would be *p. The keyword should have been delete_whatever_is_pointed_by. Similarly, free should have been called free_whatever_is_pointed_by.

FQA: It really should have been "". That's right, the keyword should have been an empty string. You don't need it. The object should live as long as someone can use it, and when it becomes unaccessible and can no longer be used by anyone, it should die. Why is making sure that this is what actually happens your job?

Of course garbage collection may be time consuming in the average and/or worst case. Are you sure your implementation of new is better in this respect though? At least with garbage collection and managed pointers you can do heap compaction. With new/delete and bare pointers, pieces of memory between used blocks too small to satisfy an actual allocation request will accumulate, and you'll effectively run out of memory. This nice situation is called external memory fragmentation. Try it with your production code that's supposed to have long (not to mention "unlimited") uptime - it's fun!

And what if you make a mistake, one single mistake with all these deletions? Either you'll run out of memory, or your program will crash, or it will corrupt its own data. Which is why many experienced C++ programmers do everything to avoid explicit calls to new. Instead, they use RAII - have some constructor do the new and the destructor do the delete. Which eventually leads to lots of unnecessary copying - you can't point to some data inside a data structure since the data structure may be about to die, and it will kill all its data whether or not someone points to it. So you have to make a copy of that data, and keep believing that manual memory management is what makes your C++ programs so fast.

If managing the life and death of objects is such a big deal, perhaps the language isn't very object-oriented after all, is it?

[16.2] Is it safe to delete the same pointer twice?

FAQ: It isn't. Don't do that. Your program may crash or corrupt its own data. If it works in a test, it doesn't mean it always works. Don't do that.

FQA: When you delete the pointer, it still points to the same place - the only difference is that the place was marked as "free" in some system-specific way. The second call to delete will probably try to mark it as "free" again. Which may be problematic when it's no longer free, actually, because someone has already allocated an object there, and you've just wiped it out with your second delete call, so now still other someone can allocate an object there and overwrite the object you've destroyed so immorally.

There are other ways for this to fail - say, the code marking blocks as "free" and assuming they are taken may count on some memory right before the place pointed by the block start pointer to contain some meta-data it no longer contains, etc.

Here's what happens in managed environments. First, you don't delete anything: the environment does. Second, it never deletes anything unless nobody points to it. So you are never stuck with pointers pointing to graves of dead objects, which may already be inhibited by freshly created objects. Which is good, because whatever your project is, memory management is not one of its stated goals, is it? It's nice not to do something you don't really want to do.

[16.3] Can I free() pointers allocated with new? Can I delete pointers allocated with malloc()?

FAQ: You can't. Don't do that. Your program may crash or corrupt its own data. If it works in a test, it doesn't mean it always works. Don't do that.

FQA: The many duplicate C++ facilities, such as new and malloc, are ugly, but using mismatching functions for allocation and deallocation is not less ugly. Why do you want to do that? It's like using an opening parenthesis and a closing bracket. How can you count on something like this to work reliably?

[16.4] Why should I use new instead of trustworthy old malloc()?

FAQ: new/delete call the constructor/destructor; new is type safe, malloc is not; new can be overridden by a class.

FQA: The virtues of new mentioned by the FAQ are not virtues, because constructors, destructors, and operator overloading are garbage (see what happens when you have no garbage collection?), and the type safety issue is really tiny here (normally you have to cast the void* returned by malloc to the right pointer type to assign it to a typed pointer variable, which may be annoying, but far from "unsafe").

Oh, and using trustworthy old malloc makes it possible to use the equally trustworthy & old realloc. Too bad we don't have a shiny new operator renew or something.

Still, new is not bad enough to justify a deviation from the common style used throughout a language, even when the language is C++. In particular, classes with non-trivial constructors will misbehave in fatal ways if you simply malloc the objects. So why not use new throughout the code? People rarely overload operator new, so it probably won't get in your way too much. And if they do overload new, you can always ask them to stop.

[16.5] Can I use realloc() on pointers allocated via new?

FAQ: Guess what - you can't. realloc may end up copying memory, and C++ objects don't like to have their memory copied without getting notified. They like to have their copy constructors and operator= handle the copying.

By the way, why do you think malloc/realloc/free use the same heap as new/delete?

FQA: The fact that there's no renew or whatever you'd call it is one of the reasons to use custom allocators instead of new. You can then allocate uninitialized memory with malloc and use placement new to call constructors. Code doing these things is usually as ugly as sin, hard to get right and gets in the way of debuggers.

Many C++ objects will live happily ever after they're moved (realloc'd). A different set of objects always get broken - those pointing to the old place. The intersection of these sets is non-empty since objects can keep pointers into themselves. Which is the special case that's sort of solved by copy constructors and operator= (the solution is simply to have you implement the copying so that pointers are set up correctly).

However, the general case can't be solved - you can't move a C++ object and destroy the old one unless you can prove that no pointers are left to the old location. This can't be proved automatically in the general case (the halting problem and all that). Which is why you can't do automatic heap compaction, which could solve the external fragmentation problem.

If your implementation uses two heaps, one for malloc and one for new, then it stinks, because sometimes you want to somehow replace malloc with something else, and it's nice to be able to do it once (for malloc), not twice (for malloc and new), and two heaps probably mean more fragmentation, and what's the point of two heaps? I've never seen this done, but it actually is legal.

[16.6] Do I need to check for NULL after p = new Fred()?

FAQ: No, that would be bad. new throws an exception, so you don't have to insert tests all over the place. Unless you're using an old compiler (you can still work around it and have an exception thrown).

FQA: C++ exceptions are frequently worse than having your code simply and straight-forwardly crash (at least in the latter case, you have a good chance to find the call stack where that happened). Disabling exception support at your latest & greatest compiler is sometimes a good idea.

If you really have to write code handling out-of-memory conditions gracefully, that's not as easy a task as simply catching a C++ exception. First, you'll probably have to avoid recursion (C++ doesn't throw stack overflow exceptions, you know, so the program can't behave gracefully when you're out of that memory). Second, what are you going to do when you're out of memory? Take into account that you'll need memory to do it; you might need to reserve some in advance.

And there aren't that many choices in most cases. You can exit with an error message instead of "hard" crashing. You can show a message saying that there's not enough memory and could the user please close some programs and then you'd retry, or maybe the user wants your program to quit? All these can be done without exceptions.

Exceptions can be sort of handy when your approach is to "abort the current operation", but exceptions are not a very good way to develop robust code, and if you actually want to deal gracefully with out-of-memory situations (you probably noticed that it's rarely done), that's way out of their league. The reason is that with exceptions, there are many ways to make subtle errors in your error handling code, and when some aspect of your application is important, you're usually better off making the handling of that aspect explicit and clear.

[16.7] How can I convince my (older) compiler to automatically check new to see if it returns NULL?

FAQ: You can pass a callback function throwing an exception to std::set_new_handler. This will work, except for global variables calling new before you set your handler. There is no way to make sure your handler is set first - even if you set it in the constructor of a global variable, that variable is not necessarily the first one to be constructed.

FQA: I wonder why you want new to throw exceptions. This desire may be the indication of a certain kind of mindset. Hmmm, let's conduct a test: are you happy with the FAQ's solution, or are you actually bothered by the fact that a constructor of a global variable may run out of memory without throwing an exception?

If this option scares you, it probably means that you like to do hairy things before main in undefined order, including throwing and catching exceptions, which confirms my worst fears. I hope you get out of these habits by the time we happen to work on the same project.

If, on the contrary, you think that global constructors shouldn't get that complicated, your approach is apparently a little bit more practical, and now I'm really puzzled. Are you absolutely sure you want new to throw exceptions?

[16.8] Do I need to check for NULL before delete p?

FAQ: delete already does that, so no, you shouldn't! You could get the test wrong, and you'll spend time testing both execution paths as required by testing methodologies.

FQA: Apparently the FAQ is written for imbeciles that can't test for NULL, and they don't test the code after writing it to see if it worked, but they do mechanically test all branches because of testing methodologies. If you want to really create a vivid image of a member of the FAQ's target audience in your imagination, think about this: just how does the idiot cover both execution paths? The poor creature must artifically create cases where p is NULL, without having the rest of the program crash. Which can be tricky enough to be inconsistent with the rest of our data about the mental capabilities of this programmer. And now we get a clear picture: the FAQ is designed for extraterrestrial intelligence struggling with the many difficulties of C++.

Actually, there is no reason to be mean this time. Once in a lifetime those people actually made something easy. Maybe they were inspired by the example of free, which also accepts null pointers. Pretty inconsistent with the spirit of the language. Which is why I was very surprised when I first found this out.

[16.9] What are the two steps that happen when I say delete p?

FAQ: When p is a T*, the first step is p->~T();, and the second is operator delete(p);.

FQA: Ugly syntax, isn't it? Especially the fact that a+b is the same as operator+(a,b), but delete p is not at all the same as operator delete(p).

The semantics are consistent with the decoupling of allocation (operator new) and construction (T()). This decoupling breaks encapsulation without admitting it (the caller must know the size of the objects of the class at compile time, which means it must know all the private members) in order to increase efficiency in straight-forward implementations (which can't do just-in-time compilation with optimization and have to know the size at compile time to optimize allocation). Which is why C++ code is recompiled all the time.

[16.10] In p = new Fred(), does the Fred memory "leak" if the Fred constructor throws an exception?

FAQ: No, because the compiler effectively generates a try/catch block around the constructor call, and when exceptions are thrown, the catch part deallocates the memory and rethrows the exception.

FQA: If you throw exceptions in constructors, make sure they are caught when custom allocators are used (the kind that looks like this: new (pool.alloc()) Fred()).

If you don't throw exceptions, the implicit try/catch around new is one illustration of the fact that exceptions increase the size of your code even when you don't use them. One good thing is that most compilers have a flag disabling exceptions.

[16.11] How do I allocate / unallocate an array of things?

FAQ: You allocate it with p = new T[N] and deallocate with delete[] p. Using delete p is an error.

FQA: You can only do that if T has a default constructor (one that can work without arguments). Which is one more reason to avoid non-trivial constructors. Alternatively, if you'd rather create a problem than solve one, you can replace your evil arrays with std::vector.

[16.12] What if I forget the [] when deleting array allocated via new T[n]?

FAQ: You'll infect your program with an incurable decease. It will do something like corrupt its data and die.

FQA: If you want to realize the idiocy of this rule in its full glory, consider this. new calls malloc or some other allocator, and it passes it the block size. The allocated pointer is then passed to delete, but the size is not. How does delete know to free a block of the right size - not too little, not too much? It has to store the size somewhere, doesn't it? But of course it can't be bothered to figure out the number of objects pointed by the pointer (like, divide the stored size by sizeof(*p), making an actual use of "type safety") so it can call the destructors properly.

But wait, there's more! What's the deal with data corruption? Shouldn't the worst possible effect be a resource leak due to the fact that some destructors are not called?

Here's the best part. operator new[] allocates a little bit more memory than it's asked for in order to store the number of the frigging objects in that place (it can also be done in other ways, but the effect is equivalent). delete[] uses it to call the destructors for all the allocated objects. Since new doesn't store any number and only allocates the amount of memory it was told to, it becomes clear why mismatching the new and delete operators will lead to data corruption.

The bottom line is this: C++ stores the number of objects in a block once when new is called and twice when new[] is called, and it will rather have you introduce lethal bugs in your program than use the information to help you get it right. Now that is what I call "hospitality".

[16.13] Can I drop the [] when deleting array of some built-in type (char, int, etc)?

FAQ: No you can't. You may think that you can, because int has a trivial destructor. But it's not just about destructors. For example, what if someone replaces operator new[] and operator delete[] in a way incompatible with mismatching new/delete[] calls?

FQA: Yeah, did you think about that? And what if you use an array of Int, which is a typedef, and someone changes the typedef to SmartIntClass instead of plain old int? Think about it. That's what your brain is for: thinking about exciting, useful things like this.

The serious answer is that if a language has two sets of similar matching operators, than a programmer is better off avoiding mismatches between those operators. The fact that there really should have been one delete operator, no, wait, make it zero, may be a reason to switch to a different language, but not to violate the rules of a language.

[16.14] After p = new Fred[n], how does the compiler know there are n objects to be destructed during delete[] p?

FAQ: Sing along: "It's a kind of magic..."

There are two common ways used by our friends the compiler-implementing magicians. One is to allocate extra memory and store the number of objects there. The other one is to use an associative array mapping from pointers to sizes.

FQA: You may wonder why these clever magicians don't rely on malloc to do the magic, and have you end up with the block size stored twice: you ask new for 8 bytes, then new asks malloc for 12 bytes, then malloc asks sbrk or whatever's down its guts for 16 bytes.

The answer is of course modularity. Translation: why should the implementor of the C++ language bother to figure out the details of malloc when there's the easier option - leave it to the implementor of the C language and simply and portably implement new[] on top of the C runtime? 4 bytes of your memory are surely not a good enough reason.

"Magic". The next thing you know, they'll call exploiting security holes in your OS to utilize some of those unused processor & memory resources "magic".

[16.15] Is it legal (and moral) for a member function to say delete this?

FAQ: You can do this, as long as you are sure the object is allocated with new, and the pointer is not used for anything at all after delete this.

FQA: This is a bit weird, and it probably causes many people to get alarmed and start anxiously looking for places that might touch the deleted object (as if it were more likely than a less exotic access to a dangling reference). And it forces the user of a class to allocate the objects with new, which is against the (misguided) spirit of C++ and therefore another source of confusion. At least provide a static method that allocates objects with new and declare the destructors private to document your intent to control the allocation.

Why do you want to do it anyway? If you want to impress people, why not do some real magic - actual functionality, not just yet another syntactic exercise?

[16.16] How do I allocate multidimensional arrays using new?

FAQ: The answer is very long with 3 large code listings.

FQA: There is no operator new[][] in C++ - it decided that new and new[] are sufficient; it had many stupid reasons to decide that way. Anyway, you have two options: allocate a flat array and manage the N-dimensional indexing manually (generating one-dimensional indexes using expressions like x+y*width), or you can allocate an array of arrays (correction). For 2 dimensions, you need to allocate a new T*[M] and then in a loop with M iterations allocate a new T[N].

With the second way you get more natural-looking indexing, but worse performance (less speed, more space). It may be convenient if you need each one-dimensional array in the two-or-more-dimensional array to have a different length, but even then it's probably better to fake it with a flatter kind of array in ugly ways if you care about performance (by the way, if you don't, make the best of it - try a safe language).

[16.17] But the previous FAQ's code is SOOOO tricky and error prone! Isn't there a simpler way?

FAQ: Sure it is! You can define a Matrix class and have it do all the dirty work. Which is good, as should be obvious to anyone familiar with Star Trek 2 (a reference follows).

FQA: I didn't see Star Trek, but moving the dirty work to a single place doesn't make that work "simple". Of course it's better than doing that work over and over again, but how many multi-dimensional array classes have you seen, and what happens when someone tries to convert between them? I mean it can't be a "single place" unless that place is the compiler.

By the way, if you need performance, you can't really encapsulate the layout of the multi-dimensional array, because you'll need to do things like get one column to the right (with flat_index+1) or one row up (with flat_index+row_width), instead of computing things like x+(y+1)*row_width every time through the innermost loop.

You can try to define all kinds of functions like increment_by_row. If you really believe that it will make it possible to "transparently" change the representation later (thus turning all the optimizations carefully developed for this representation into pessimizations), go ahead and define and optimize hordes of such tiny functions and keep lying to yourself about how this makes your code more readable.

Data representation is very important. Too bad so many people believe that the only important thing about data is to hide it behind code (interfaces) so that you can "always change the data representation later". While this may be right in the majority of cases, it is likely to be wrong in the most important cases.

[16.18] But the above Matrix class is specific to Fred! Isn't there a way to make it generic?

FAQ: Of course! You can use templates!

FQA: This way, not only will your code run slowly - it will also compile slowly.

[16.19] What's another way to build a Matrix template?

FAQ: You can use std::vector instead of bare pointers!

FQA: You can also use any of the numerous array classes implemented before std::vector was introduced into the C++ standard library (which was long after C++ became widely used).

Alternatively, you can also use any of the numerous matrix classes implemented before std::matrix was introduced into the C++ standard library... Actually, make that "before std::matrix will be introduced into the C++ standard library". Can you explain why the C++ standard includes std::vector - a class quite similar to built-in arrays, but different - but does not include std::matrix, a class that would be quite similar to built-in 2D arrays, but different? Here's a more difficult question: is it good or bad that we don't have std::matrix, provided that we do have less than optimal built-in 2D arrays? Before you answer, consider type conversions, dynamic resizing, literal values, telling the size of an array at run time...

That's what C++ is all about: the development of your ethical judgment. Everybody can tell good from bad; choosing between the ugly and the disgusting takes real wisdom.

[16.20] Does C++ have arrays whose length can be specified at run-time?

FAQ: That would be yes - there's std::vector.

Actually, it's a no - you can't do it with built-in arrays.

Wait a minute - you can do it with the first index of a built-in array, for example: new T[run_time_val][compile_time_val].

You know what? Arrays are evil.

FQA: std::vector is not an array, it's a class that looks like an array until you try to initialize it with {1,2,3} or pass an array to a function expecting a vector or get a huge compiler error message. Built-in arrays don't behave like this.

C++ inherits the following rule about types from C: the size of a type is always known at compile time, and you can get it with sizeof. This rule is useful in C because, together with other rules which are violated in C++, it makes it quite easy to mentally map source code to assembly code (sort of) and estimate run time performance. In C++ it's no longer useful, because telling the meaning of C++ code is nearly impossible, not to mention reasoning about its performance.

Anyway, the length of a built-in array is part of its type, so if it could be dynamic, it would violate the ex-useful rule. So it can't. However, built-in pointers can point to an array of length defined at run time. So you can have a T* p pointing to run_time_val objects, but you can't have an array of type T[run_time_val]. Which means that you can't allocate such arrays on the stack (unless the compiler decides to almost violate the ex-useful rule and support it, the way GNU C/C++ does, for example).

The built-in C++ arrays are inherited from C without changes (well, except for constructor/destructor calls and new in addition to malloc and other stuff of this kind), so if you care about details, the best place to look is material about C. This is one of the many examples illustrating that you have to know C in order to really understand C++.

[16.21] How can I force objects of my class to always be created via new rather than as locals or global/static objects?

FAQ: You can define the constructors private and provide public functions which return pointers to objects allocated with new. Which goes by the fancy name of "The Named Constructor Idiom".

FQA: Why do you want to force this? Allocating locals is more efficient than new, and you wouldn't have to worry about delete, either. The cost is that when you change the private members of the class, all code using it gets recompiled.

Wait, is that what you want - to avoid recompilation? In that case, the "Named Constructor Idiom" defeats the purpose - change a private member and you still trigger a rebuild. You can either drop C++ classes and store the state in a C struct (place a forward declaration in the header file and the full definition in the implementation file), or you can wrap this exact solution with the code of an extra C++ class just because C is evil. The latter way is known as "The Pimpl Idiom" ("pimpl" apparently stands for "pointer to implementation").

Idioms make the world move. Especially the Design Pattern Idiom. Or the Idiom Design Pattern. Or something.

[16.22] How do I do simple reference counting?

FAQ: Two large code listings are given (a quote from these listings: // DO NOT CHANGE THE ORDER OF THESE STATEMENTS!), followed by a bold claim that now you have a way to do simple reference counting.

FQA: You do simple reference counting by using a language which does reference counting (or garbage collection - you probably don't care if you want it "simple").

You do complicated and broken reference counting by using C++, creating smart pointer classes (and smart array classes, and smart multi-dimensional array classes), wrapping your private constructors with public functions returning smart pointers to objects (or smart arrays of objects) to make sure all objects are actually pointed by smart pointers so the reference counting is not entirely worthless.

Congratulations! You've just wasted lots and lots of time to emulate garbage collection on top of C++. Well, the code doesn't compile as fast as it would with built-in garbage collection (all those smart pointer templates), and the error messages are a bit cryptic (all those smart pointer templates), and there are those tricky cases like cyclic references you still fail to deal with (despite all those smart pointer templates), and for every class you write extra code wrapping constructors, and the whole thing doesn't play well with existing libraries (which use different smart pointer templates), and...

Why do you want to do reference counting in C++?

[16.23] How do I provide reference counting with copy-on-write semantics?

FAQ: You can have your class keep all the data in a structure pointed by a member pointer _data, and then each method that wants to modify the object has to check the reference count, and if the data is shared by several objects, the data must be cloned before the modification.

FQA: Ultimately, what are you trying to achieve? If you care a lot about efficiency and you care a lot about high level of abstraction in the same piece of code, there's probably no easy solution for you. Are you sure it's the same code? Or maybe you want efficiency in some places, and high level of abstraction in other places? Then you can use two different languages.

If it's really the same code, the best solution is probably to write your own little language. It's not as hard as it sounds, and not so strange - think about it: apparently you must write a lot of code (if there wasn't a lot of it, it wouldn't be a problem) in some pretty specific domain, because there's no high level language which supports the right built-in facilities. Or is there? Are you looking for copy-on-write strings or something? String processing in C++? Do yourself a favor and stop right there. Use a language with decent built-in strings.

If there really is no good existing high-level language for your job, basically what you have to do is meta-programming: you want something which is not directly related to the specific meaning of your program - you want objects of pretty much arbitrary classes to behave in a certain way. Which means you want a new language. You can do it with your own compiler or you can do it in a system which supports meta-programming well (the two approaches are really pretty close). C++ meta-programming is a nightmare - the facilities for it are very poor, and there's lots of strange features already in the language that you must interoperate with. You have been warned: this path leads to the gates of madness.

Clarification: I'm not saying that meta-programming is likely to be the solution when you want copy-on-write. On average, that would probably be over-engineering. However, in the cases where it indeed is over-engineering, so is the FAQ's solution; the way to go is probably to have the user code explicitly copy objects upon modifications. And when there's enough code involved to make that tedious, then the C++ solution also gets tedious, and that's when meta-programming could be appropriate.

[16.24] How do I provide reference counting with copy-on-write semantics for a hierarchy of classes?

FAQ: You do it by writing a lot of code! Like this:

  lots of code;
  more code;
  two screens of code;
  //boy is this FUN!

FQA: Here's a solution for another hierarchy of classes (you might have more than one in your project - it happens, and the FAQs "solution" is done on a per-method basis, and there isn't even a way to iterate over the methods of a class in C++, not to mention "transparently instrument the methods of a class with custom logic like copy-on-write"):

  //why am I doing this?
  //I think I'd rather become a farmer

Seriously, I'm not going to discuss the nightmare that is the FAQ's proposed solution. Follow the link to the FAQ's answer if you want to check it out. What I would like to point out is that it's possible to do all this transparently if your environment has good support for meta-programming. For example, some languages let you write your own code to implement object attribute modification (the simpler option), and/or automatically generate wrapper classes to intercept methods changing objects (the more complicated option). Either way, you end up with O(1) code to solve the problem instead of O(N), where N is the number of classes involved.

[16.25] Can you absolutely prevent people from subverting the reference counting mechanism, and if so, should you?

FAQ: You can't, and you usually shouldn't. (Yes, that's pretty much what the FAQ says: sometimes you should do what you can't. Just what does "should" mean in its warped universe?)

There are two holes in the armor. First, your SmartPtr probably has an operator* that returns a bare reference to an object. So you can end up with Dumb* p = &*smart_p;. This can be sort of closed using several approaches, each worse than the others in its own unique way. And then there are ways to get the bare pointer with syntax like smart_p.operator->(), returning a Dumb*.

The second hole is that someone can have dangling references to SmartPtr. This can't be prevented even by returning a SmartPtrPtr in an overloaded operator&, because C++ has references (remember - the new feature you should use instead of the evil pointers), and there's nothing you can overload to prevent someone from taking a reference to your SmartPtr.

Use code reviews to figure out what's actually going on with all those smart pointers.

FQA: You can't fake what you don't have (I think the quote is attributed to S. Cray). C++ doesn't have safe memory management. Faking safe memory management leads to still unsafe memory management, with the important bonus of obfuscation.

You can absolutely prevent people from subverting the reference counting mechanism by using a programming language that absolutely prevents people from subverting its built-in memory management mechanisms.

[16.26] Can I use a garbage collector in C++?

FAQ: Sure you can. Let's compare it to smart pointer techniques. Garbage collection is not as portable, but typically more efficient, and it can handle cyclic references, and it works better with others' libraries because you don't need to explicitly change the pointers from dumb to smart or anything like it. However, sometimes objects can leak - a C++ garbage collector can't tell a pointer from a random bit sequence that just happens to look like a pointer.

FQA: Assuming you live in a free country, what's there to stop you from using a garbage collector in C++? Sure, there are minor obstacles, for example, it doesn't really work, but it's still a free country, isn't it?

Suppose you can use garbage collection, which means that you are not worried about interference with real time requirements. Why are you using C++? Why not use a safe language instead? Look - the C++ FAQ admits that garbage collection is more efficient than C++ "smart pointers" all over the place, and more correct (cyclic references), and the single correctness problem it mentions (random bit patterns) vanishes in a managed environment. So what's the problem?

If you think you need to use C++, deal with the fact that it doesn't have garbage collection, and you must manage memory manually. Or you can deal with the other facts. For example, what happens if two libraries relying on different garbage collectors are supposed to work in the same program? What happens if code incompatible with garbage collection (because it does "clever" things with pointers, like one_based_array = (int*)malloc(arr_size)-1)? What about destructors? C++ garbage collectors don't know what they're freeing, so you can't have finalization functions. What about all those happy custom allocators C++ programmers adore so much? What if a malloced object points to an object allocated from such a custom pool? The destructor of the outer object with the pointer is not going to be called - so who is going to deallocate the pointed object from the pool?

C++ does not feel pain. It can't be reasoned with. Starting a fight is a big mistake. If you want to use C++, you must learn to love it the way it is, in particular, manage your memory manually.

[16.27] What are the two kinds of garbage collectors for C++?

FAQ: There are conservative and hybrid garbage collectors. The conservative ones just look for bit sequences looking like pointers. The hybrid ones require the programmer to specify some layout information explicitly in the code (but they still traverse the call stack conservatively when they look for pointers).

Garbage collectors may cause memory leaks when a bit pattern looking like a pointer is misinterpreted as a proof that the block pointed by this "pointer" is still in use. And some illegal programs may "confuse" garbage collectors (that's the word used by the FAQ) by keeping pointers outside of allocated blocks. Why can't these programmers behave reasonably?

FQA: Let's start with a different question: what's the problem with garbage collection in C++? Answer: there is no way to inspect the state of a C++ program and tell which blocks are in use. In environments designed to support garbage collection, you can do that by checking if the program can reach the block using one of the pointers it currently keeps in its memory. But in C++, you can't. First, you don't know where the program keeps pointers (no real run time type information), and second, pointers that don't point into a block can be used to compute pointers that do point into that block (unsafe pointer arithmetics).

So along come the memory leaks, and the "confusion". And what happens when a garbage collection is "confused"? I'll tell you what happens - it frees an object which is still in use. The result is a crash or a data corruption. Do you think "confused" is a legitimate euphemism in this context? "Oh, I'm so confused! I think I'll kill your program now."

As to the "unreasonable" programmers - well, adjusting a pointer to point before an allocated block is one efficient way to implement one-based (instead of zero-based) arrays, among other things. Of course you can allocate an extra element and avoid violating the rules. But most people don't know these rules, because although they are a part of the language definition, they are not enforced in the widespread implementations, and don't become a part of the mental model developed by programmers as they gain familiarity with the language.

That is, not only does it work when one implements one-based arrays this way - one doesn't see a reason for it not to work: experience consistently tells people that C++ pointers are just a kind of integer. Formally, there are reasons for this to be illegal (like, duh, what happens if you want garbage collection?) - but most people are not language lawyers. In practice, what matters is the de facto standard (that's why it's called "de facto").

[16.28] Where can I get more info on garbage collectors for C++?

FAQ: Here.

FQA: Don't bother. If garbage collection worked in C++, you would hear about it. C++ has been around for decades, there are hundreds of thousands of C++ programmers, and megatons of C++ code with zillions of memory management bugs. If someone implemented a working solution for this problem, they'd become rich, famous, or both, and the garbage collector(s) would be everywhere.

Why do you think you don't see too many around?

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009