Garbage Collection in C++ -- why?

Question

I keep hearing people complaining that C++ doesn't have garbage collection. I also hear that the C++ Standards Committee is looking at adding it to the language. I'm afraid I just don't see the point to it... using RAII with smart pointers eliminates the need for it, right?

My only experience with garbage collection was on a couple of cheap eighties home computers, where it meant that the system would freeze up for a few seconds every so often. I'm sure it has improved since then, but as you can guess, that didn't leave me with a high opinion of it.

What advantages could garbage collection offer an experienced C++ developer?

Answer 1

The committee isn't adding garbage-collection, they are adding a couple of features that allow garbage collection to be more safely implemented. Only time will tell whether they actually have any effect whatsoever on future compilers. The specific implementations could vary widely, but will most likely involve reachability-based collection, which could involve a slight hang, depending on how it's done.

One thing is, though, no standards-conformant garbage collector will be able to call destructors - only to silently reuse lost memory.

Answer 2

I don't know much about the future of C++ but reasons for garbage collection include:

(From mostly wikipedia)

• Pointers to garbage data
• Data that doesn't get freed after pointer is destroyed (memory leak)
• Freeing memory that has already been freed

http://en.wikipedia.org/wiki/Automatic_garbage_collection

Answer 3

The short answer is that garbage collection is very similar in principle to RAII with smart pointers. If every piece of memory you ever allocate lies within an object, and that object is only referred to by smart pointers, you have something close to garbage collection (potentially better). The advantage comes from not having to be so judicious about scoping and smart-pointering every object, and letting the runtime do the work for you.

This question seems analogous to "what does C++ have to offer the experienced assembly developer? instructions and subroutines eliminate the need for it, right?"

Answer 4

With the advent of good memory checkers like valgrind, I don't see much use to garbage collection as a safety net "in case" we forgot to deallocate something - especially since it doesn't help much in managing the more generic case of resources other than memory (although these are much less common). Besides, explicitly allocating and deallocating memory (even with smart pointers) is fairly rare in the code I've seen, since containers are a much simpler and better way usually.

But garbage collection can offer performance benefits potentially, especially if alot of short lived objects are being heap allocated. GC also potentially offers better locality of reference for newly created objects (comparable to objects on the stack).

Answer 5

I, too, have doubts that C++ commitee is adding a full-fledged garbage collection to the standard.

But I would say that the main reason for adding/having garbage collection in modern language is that there are too few good reasons against garbage collection. Since eighties there were several huge advances in the field of memory management and garbage collection and I believe there are even garbage collection strategies that could give you soft-real-time-like guarantees (like, "GC won't take more than .... in the worst case").

Answer 6

I keep hearing people complaining that C++ doesn't have garbage collection.

I am so sorry for them. Seriously.

C++ has RAII, and I always complain to find no RAII (or a castrated RAII) in Garbage Collected languages.

What advantages could garbage collection offer an experienced C++ developer?

Another tool.

Matt J wrote it quite right in his post (http://stackoverflow.com/questions/228620/garbage-collection-in-c-why#228640): We don't need C++ features as most of them could be coded in C, and we don't need C features as most of them could coded in Assembly, etc.. C++ must evolve.

As a developper: I don't care about GC. I tried both RAII and GC, and I find RAII vastly superior. As said by Greg Rogers in his post (http://stackoverflow.com/questions/228620/garbage-collection-in-c-why#228670), memory leaks are not so terrible (at least in C++, where they are rare if C++ is really used) as to justify GC instead of RAII. GC has non deterministic deallocation/finalization and is just a way to write a code that just don't care with specific memory choices.

This last sentence is important: It is important to write code that "juste don't care". In the same way in C++ RAII we don't care about ressource freeing because RAII do it for us, or for object initialization because constructor do it for us, it is sometimes important to just code without caring about who is owner of what memory, and what kind pointer (shared, weak, etc.) we need for this or this piece of code. There seems to be a need for GC in C++. (even if I personaly fail to see it)

An example of good GC use in C++

Sometimes, in an app, you have "floating data". Imagine a tree-like structure of data, but no one is really "owner" of the data (and no one really cares about when exactly it will be destroyed). Multiple objects can use it, and then, discard it. You want it to be freed when no one is using it anymore.

The C++ approach is using a smart pointer. The boost::shared_ptr comes to mind. So each piece of data is owned by its own shared pointer. Cool. The problem is that when each piece of data can refer to another piece of data. You cannot use shared pointers because they are using a reference counter, which won't support circular references (A points to B, and B points to A). So you must know think a lot about where to use weak pointers (boost::weak_ptr), and when to use shared pointers.

With a GC, you just use the tree structured data.

The downside being that you must not care when the "floating data" will really be destroyed. Only that it will be destroyed.

Conclusion

So in the end, if done properly, and compatible with the current idioms of C++, GC would be a Yet Another Good Tool for C++.

C++ is a multiparadigm language: Adding a GC will perhaps make some C++ fanboys cry because of treason, but in the end, it could be a good idea, and I guess the C++ Standards Comitee won't let this kind of major feature break the language, so we can trust them to make the necessary work to enable a correct C++ GC that won't interfere with C++: As always in C++, if you don't need a feature, don't use it and it will cost you nothing.

Answer 7

Garbage collection is really the basis for automatic resource management. And having GC changes the way you tackle problems in a way that is hard to quantify. For example when you are doing manual resource management you need to:

• Consider when an item can be freed (are all modules/classes finished with it?)
• Consider who's responsibility it is to free a resource when it is ready to be freed (which class/module should free this item?)

In the trivial case there is no complexity. E.g. you open a file at the start of a method and close it at the end. Or the caller must free this returned block of memory.

Things start to get complicated quickly when you have multiple modules that interact with a resource and it is not as clear who needs to clean up. The end result is that the whole approach to tackling a problem includes certain programming and design patterns which are a compromise.

In languages that have garbage collection you can use a disposable pattern where you can free resources you know you've finished with but if you fail to free them the GC is there to save the day.

---

Smart pointers which is actually a perfect example of the compromises I mentioned. Smart pointers can't save you from leaking cyclic data structures unless you have a backup mechanism. To avoid this problem you often compromise and avoid using a cyclic structure even though it may otherwise be the best fit.

Answer 8

Garbage collection allows to postpone the decision about who owns an object. With RAII, indeed, objects are recollected when going out of scope. This is sometimes referred to as "immediate GC".

The tricky thing about GC is deciding upon when an object is no longer needed.

Answer 9

I don't understand how one can argue that RAII replaces GC, or is vastly superior. There are many cases handled by a gc that RAII simply cannot deal with at all. They are different beasts.

First, RAII is not bullet proof: it works against some common failures which are pervasive in C++, but there are many cases where RAII does not help at all; it is fragile to asynchronous events (like signals under UNIX). Fundamentally, RAII relies on scoping: when a variable is out of scope, it is automatically freed (assuming the destructor is correctly implemented of course).

Here is a simple example where neither auto_ptr or RAII can help you:

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <memory>

using namespace std;

volatile sig_atomic_t got_sigint = 0;

class A {
        public:
                A() { printf("ctor\n"); };
                ~A() { printf("dtor\n"); };
};

void catch_sigint (int sig)
{
        got_sigint = 1;
}

/* Emulate expensive computation */
void do_something()
{
        sleep(3);
}

void handle_sigint()
{
        printf("Caught SIGINT\n");
        exit(EXIT_FAILURE);
}

int main (void)
{
        A a;
        auto_ptr<A> aa(new A);

        signal(SIGINT, catch_sigint);

        while (1) {
                if (got_sigint == 0) {
                        do_something();
                } else {
                        handle_sigint();
                        return -1;
                }
        }
}

The destructor of A will never be called. Of course, it is an artificial and somewhat contrived example, but a similar situation can actually happen; for example when your code is called by another code which handles SIGINT and which you have no control over at all (concrete example: mex extensions in matlab). It is the same reason why finally in python does not guarantee execution of something. Gc can help you in this case.

Other idioms do not play well with this: in any non trivial program, you will need stateful objects (I am using the word object in a very broad sense here, it can be any construction allowed by the language); if you need to control the state outside one function, you can't easily do that with RAII (which is why RAII is not that helpful for asynchronous programming). OTOH, gc have a view of the whole memory of your process, that is it knows about all the objects it allocated, and can clean asynchronously.

It can also be much faster to use gc, for the same reasons: if you need to allocate/deallocate many objects (in particular small objects), gc will vastly outperform RAII, unless you write a custom allocator, since the gc can allocate/clean many objects in one pass. Some well known C++ projects use gc, even where performance matter (see for example Tim Sweenie about the use of gc in Unreal Tournament: http://lambda-the-ultimate.org/node/1277). GC basically increases throughput at the cost of latency.

Of course, there are cases where RAII is better than gc; in particular, the gc concept is mostly concerned with memory, and that's not the only ressource. Things like file, etc... can be well handled with RAII. Languages without memory handling like python or ruby do have something like RAII for those cases, BTW (with statement in python). RAII is very useful when you precisely need to control when the ressource is freed, and that's quite often the case for files or locks for example.

Answer 10

What advantages could garbage collection offer an experienced C++ developer?

Not having to chase down resource leaks in your less-experienced colleagues' code.

Answer 11

The motivating factor for GC support in C++ appears to be lambda programming, anonymous functions etc. It turns out that lambda libraries benefit from the ability to allocate memory without caring about cleanup. The benefit for ordinary developers would be simpler, more reliable and faster compiling lambda libraries.

GC also helps simulate infinite memory; the only reason you need to delete PODs is that you need to recycle memory. If you have either GC or infinite memory, there is no need to delete PODs anymore.

Answer 12

It's an all-to-common error to assume that because C++ does not have garbage collection baked into the language, you can't use garbage collection in C++ period. This is nonsense. I know of elite C++ programmers who use the Boehm collector as a matter of course in their work.

Answer 13

I like RAII and smart pointers.

1. I don't like to declare types whenever I use a smart pointer.
2. I want to be able to collect intermediate values so I can write compact code.

I designed GCPtr class specifically for the case of intermediate values:

//  gcptr.h
//  Created by "tydok" on 18-03-2009

#ifndef GARBAGE_COLLECT_POINTER_H__
#define GARBAGE_COLLECT_POINTER_H__

/* Syntactic sugar for automatic objects.
   Instead of GCPtr()(pointer) -> GCPTR(pointer)  */
#define GCPTR   GCPtr()

class GCPtr {
  // Prevent copying and assignment; not implemented
  GCPtr( const GCPtr& );
  GCPtr* operator=( const GCPtr& );

  void* _ptr;
  void* (GCPtr::*_pdel)(void*);

public:
  GCPtr() {
    _ptr = 0;
    _pdel = 0;
  }
  ~GCPtr() {
    if (_pdel && _ptr) (this->*_pdel)(0);
  }

  template<typename T>
  T operator()(T ptr) {
    if (ptr != 0) goto NORMAL;
    if (_ptr) delete (T)_ptr;
    _ptr = 0;
    return 0;
    NORMAL:
    _pdel = (void* (GCPtr::*)(void*))((T (GCPtr::*)(T))operator());
    _ptr = ptr; // <--- Any previous _ptr value will be lost
    return (T)(_ptr);
  }
};

#endif

Usage example:

#include <malloc.h>
#include <string.h>
#include <stdio.h>
#include "gcptr.h"

void
print_reversed_string( char* s )
{
  // Version 1, the usual way
  char* rs = _strrev(_strdup(s));
  printf("%s\n", rs);
  free(rs);

  // Version 2, using GCPtr
  // The allocated string will be deleted right after
  // printf is done.
  printf("%s\n", GCPTR(_strrev(_strdup(s))));
  // or
  //printf("%s\n", _strrev(GCPTR(_strdup(s))));
}

GCPtr's mechanism is simple. It stores the passed in pointer until it is time for the GCPtr object to be destroyed and it is when the pointer will be deallocated.

GCPtr works for objects allocated with malloc and new. You can, of course, use as many auto GCPtr objects in function calls as you need.

Answer 14

There is one property of GC which may be very important in some scenarios. Assignment of pointer is naturally atomic on most platforms, while creating thread-safe reference counted ("smart") pointers is quite hard and introduces significant synchronization overhead. As a result, smart pointers are often told "not to scale well" on multi-core architecture.