RAII与垃圾收集器

Question

I recently watched a great talk by Herb Sutter about "Leak Free C++..." at CppCon 2016 where he talked about using smart pointers to implement RAII (Resource acquisition is initialization) - Concepts and how they solve most of the memory leaks issues.

Now I was wondering. If I strictly follow RAII rules, which seems to be a good thing, why would that be any different from having a garbage collector in C++? I know that with RAII the programmer is in full control of when the resources are freed again, but is that in any case beneficial to just having a garbage collector? Would it really be less efficient? I even heard that having a garbage collector can be more efficient, as it can free larger chunks of memory at a time instead of freeing small memory pieces all over the code.

Answer 1

If I strictly follow RAII rules, which seems to be a good thing, why would that be any different from having a garbage collector in C++?

While both deal with allocations, they do so in completely different manners. If you are reffering to a GC like the one in Java, that adds its own overhead, removes some of the determinism from the resource release process and handles circular references.

You can implement GC though for particular cases, with much different performance characteristics. I implemented one once for closing socket connections, in a high-performance/high-throughput server (just calling the socket close API took too long and borked the throughput performance). This involved no memory, but network connections, and no cyclic dependency handling.

I know that with RAII the programmer is in full control of when the resources are freed again, but is that in any case beneficial to just having a garbage collector?

This determinism is a feature that GC simply doesn't allow. Sometimes you want to be able to know that after some point, a cleanup operation has been performed (deleting a temporary file, closing a network connection, etc).

In such cases GC doesn't cut it which is the reason in C# (for example) you have the IDisposable interface.

I even heard that having a garbage collector can be more efficient, as it can free larger chunks of memory at a time instead of freeing small memory pieces all over the code.

Can be ... depends on the implementation.

Answer 2

Garbage collection solves certain classes of resource problems that RAII cannot solve. Basically, it boils down to circular dependencies where you do not identify the cycle before hand.

This gives it two advantages. First, there are going to be certain types of problem that RAII cannot solve. These are, in my experience, rare.

The bigger one is that it lets the programmer be lazy and not care about memory resource lifetimes and certain other resources you don't mind delayed cleanup on. When you don't have to care about certain kinds of problems, you can care more about other problems. This lets you focus on the parts of your problem you want to focus on.

The downside is that without RAII, managing resources whose lifetime you want constrained is hard. GC languages basically reduce you to either having extremely simple scope-bound lifetimes or require you to do resource management manually, like in C, with manually stating you are done with a resource. Their object lifetime system is strongly tied to GC, and doesn't work well for tight lifetime management of large complex (yet cycle-free) systems.

To be fair, resource management in C++ takes a lot of work to do properly in such large complex (yet cycle-free) systems. C# and similar languages just make it a touch harder, in exchange they make the easy case easy.

Most GC implementations also forces non-locality full fledged classes; creating contiguous buffers of general objects, or composing general objects into one larger object, is not something that most GC implementations make easy. On the other hand, C# permits you to create value type struct s with somewhat limited capabilities. In the current era of CPU architecture, cache friendliness is key, and the lack of locality GC forces is a heavy burden. As these languages have a bytecode runtime for the most part, in theory the JIT environment could move commonly used data together, but more often than not you just get a uniform performance loss due to frequent cache misses compared to C++.

The last problem with GC is that deallocation is indeterminate, and can sometimes cause performance problems. Modern GCs make this less of a problem than it has been in the past.

Answer 3

Notice that RAII is a programming idiom, while GC is a memory management technique. So we are comparing apples with oranges.

But we can restrict RAII to its memory management aspects only and compare that to GC techniques.

The main difference between so called RAII based memory management techniques (which really means reference counting , at least when you consider memory resources and ignore the other ones such as files) and genuine garbage collection techniques is the handling of circular references (for cyclic graphs ).

With reference counting, you need to code specially for them (using weak references or other stuff).

In many useful cases (think of std::vector<std::map<std::string,int>> ) the reference counting is implicit (since it can only be 0 or 1) and is practically omitted, but the contructor and destructor functions (essential to RAII) behave as if there was a reference counting bit (which is practically absent). In std::shared_ptr there is a genuine reference counter. But memory is still implicitly manually managed (with new and delete triggered inside constructors and destructors), but that "implicit" delete (in destructors) gives the illusion of automatic memory management. However, calls to new and delete still happen (and they cost time).

BTW the GC implementation may (and often does) handle circularity in some special way, but you leave that burden to the GC (eg read about the Cheney's algorithm ).

Some GC algorithms (notably generational copying garbage collector) don't bother releasing memory for individual objects, it is release en masse after the copy. In practice the Ocaml GC (or the SBCL one) can be faster than a genuine C++ RAII programming style (for some , not all, kind of algorithms).

Some GC provide finalization (mostly used to manage non-memory external resources like files), but you'll rarely use it (since most values consume only memory resources). The disadvantage is that finalization does not offer any timing guarantee. Practically speaking, a program using finalization is using it as a last resort (eg closing of files should still happen more or less explicitly outside of finalization, and also with them).

You still can have memory leaks with GC (and also with RAII, at least when used improperly), eg when a value is kept in some variable or some field but will never be used in the future. They just happen less often.

I recommend reading the garbage collection handbook .

In your C++ code, you might use Boehm's GC or Ravenbrook's MPS or code your own tracing garbage collector . Of course using a GC is a tradeoff (there are some inconvenience, eg non-determinism, lack of timing guarantees, etc...).

I don't think that RAII is the ultimate way of dealing with memory in all cases. In several occasions, coding your program in a genuinely and efficiently GC implementations (think of Ocaml or SBCL) can be simpler (to develop) and faster (to execute) than coding it with fancy RAII style in C++17. In other cases it is not. YMMV.

As an example, if you code a Scheme interpreter in C++17 with the fanciest RAII style, you would still need to code (or use) a explicit GC inside it (because a Scheme heap has circularities). And most proof assistants are coded in GC-ed languages, often functional ones, (the only one I know which is coded in C++ is Lean ) for good reasons.

BTW, I'm interested in finding such a C++17 implementation of Scheme (but less interested in coding it myself), preferably with some multi-threading ability.

Answer 4

RAII and GC solve problems in completely different directions. They are completely different, despite what some would say.

Both address the issue that managing resources is hard. Garbage Collection solves it by making it so that the developer doesn't need to pay as much attention to managing those resources. RAII solves it by making it easier for developers to pay attention to their resource management. Anyone who says they do the same thing has something to sell you.

If you look at recent trends in languages, you're seeing both approaches being used in the same language because, frankly, you really need both sides of the puzzle. You're seeing lots of languages which use garbage collection of sorts so that you don't have to pay attention to most objects, and those languages also offer RAII solutions (such as python's with operator) for the times you really want to pay attention to them.

• C++ offers RAII through constructors/destructors and GC through shared_ptr (If I may make the argument that refcounting and GC are in the same class of solutions because they're both designed to help you not need to pay attention to lifespan)
• Python offers RAII through with and GC through a refcounting system plus a garbage collector
• C# offers RAII through IDisposable and using and GC through a generational garbage collector

The patterns are cropping up in every language.

Answer 5

One of the problem about garbage collectors is that it's hard to predict program performance.

With RAII you know that in exact time resource will go out of scope you will clear some memory and it will take some time. But if you are not a master of garbage collector settings you cannot predict when cleanup will happen.

For example: cleaning a bunch of small objects can be done more effectively with GC because it can free large chunk, but it will be not fast operation, and it's hard to predict when in will occur and because of "large chunk cleanup" it will take some processor time and can affect your program performance.

Answer 6

Roughly speaking. The RAII idiom may be better for the latency and jitter . A garbage collector may be better for the system's throughput .

Answer 7

"Efficient" is a very broad term, in sense of development efforts RAII is typically less efficient than GC, but in terms of performance GC is typically less efficient than RAII. However it is possible to provide contr-examples for both cases. Dealing with generic GC when you have very clear resource (de)allocation patters in managed languages can be rather troublesome, just like the code using RAII can be surprisingly inefficient when shared_ptr is used for everything for no reason.

Answer 8

The main part of the question about whether one or the other is "beneficial" or more "efficient" cannot be answered without giving lots of context and arguing about the definitions of these terms.

_{Beyond that, you can basically feel the tension of the ancient "Is Java or C++ the better language?" flamewar crackling in the comments. I wonder what an "acceptable" answer to this question could look like, and am curious to see it eventually.}

But one point about a possibly important conceptual difference has not yet been pointed out: With RAII, you are tied to the thread that calls the destructor. If your application is single threaded (and even though it was Herb Sutter who stated that The Free Lunch Is Over : Most software today effectively still is single-threaded), then a single core may be busy with handling the cleanups of objects that are no longer relevant for the actual program...

In contrast to that, the garbage collector usually runs in its own thread, or even multiple threads, and is thus (to some extent) decoupled from the execution of the other parts.

(Note: Some answers already tried to point out application patterns with different characteristics, mentioned efficiency, performance, latency and throughput - but this specific point was not mentioned yet)

Answer 9

Garbage collection and RAII each support one common construct for which the other is not really suitable.

In a garbage-collected system, code may efficiently treat references to immutable objects (such as strings) as proxies for the data contained therein; passing around such references is almost as cheap as passing around "dumb" pointers, and is faster than making a separate copy of the data for each owner, or trying to track ownership of a shared copy of the data. In addition, garbage-collected systems make it easy to create immutable object types by writing a class which creates a mutable object, populating it as desired, and providing accessor methods, all while refraining from leaking references to anything that might mutate it once the constructor finishes. In cases where references to immutable objects need to be widely copied but the objects themselves don't, GC beats RAII hands down.

On the other hand, RAII is excellent at handling situations where an object needs to acquire exclusive services from outside entities. While many GC systems allow objects to define "Finalize" methods and request notification when they are found to be abandoned, and such methods may sometimes manage to release outside services that are no longer needed, they are seldom reliable enough to provide a satisfactory way of ensuring timely release of outside services. For management of non-fungible outside resources, RAII beats GC hands down.

The key difference between the cases where GC wins versus those where RAII wins is that GC is good at managing fungible memory that can be freed on an as-needed basis, but poor at handling non-fungible resources. RAII is good at handling objects with clear ownership, but bad at handling ownerless immutable data holders which have no real identity apart from the data they contain.

Because neither GC nor RAII handles all scenarios well, it would be helpful for languages to provide good support for both of them. Unfortunately, languages which focus on one tend to treat the other as an afterthought.

Answer 10

RAII uniformly deals with anything that is describable as a resource. Dynamic allocations are one such resource, but they are by no means the only one, and arguably not the most important one. Files, sockets, database connections, gui feedback and more are all things that can be managed deterministically with RAII.

GCs only deal with dynamic allocations, relieving the programmer of worrying about the total volume of allocated objects over the lifetime of the program (they only have to care about the peak concurrent allocation volume fitting)

Answer 11

RAII and garbage collection are intended to solve different problems.

When you use RAII you leave an object on the stack which sole purpose is to clean up whatever it is you want managed (sockets, memory, files, etc.) on leaving the scope of the method. This is for exception-safety , not just garbage collection, which is why you get responses about closing sockets and freeing mutexes and the like. (Okay, so no one mentioned mutexes besides me.) If an exception is thrown, stack-unwinding naturally cleans up the resources used by a method.

Garbage collection is the programmatic management of memory, though you could "garbage-collect" other scarce resources if you'd like. Explicitly freeing them makes more sense 99% of the time. The only reason to use RAII for something like a file or socket is you expect the use of the resource to be complete when the method returns.

Garbage collection also deals with objects that are heap-allocated , when for instance a factory constructs an instance of an object and returns it. Having persistent objects in situations where control must leave a scope is what makes garbage collection attractive. But you could use RAII in the factory so if an exception is thrown before you return, you don't leak resources.

Answer 12

I even heard that having a garbage collector can be more efficient, as it can free larger chunks of memory at a time instead of freeing small memory pieces all over the code.

That's perfectly doable - and, in fact, is actually done - with RAII (or with plain malloc/free). You see, you don't necessarily always use the default allocator, which deallocates piecemeal only. In certain contexts you use custom allocators with different kinds of functionality. Some allocators have the in-built ability of freeing everything in some allocator region, all at once, without having to iterate individual allocated elements.

Of course, you then get into the question of when to deallocate everything - whether the use of those allocators (or the slab of memory with which they're associated has to be RAIIed or not, and how.