Question

Background: When reading Dr. Stroustrup’s papers and FAQs, I notice some strong “opinions” and great advices from legendary CS scientist and programmer. One of them is about shared_ptr in C++0x. He starts explaining about shared_ptr and how it represents shared ownership of the pointed object. At the last line, he says and I quote:

. A shared_ptr represents shared
ownership but shared ownership isn’t
my ideal: It is better if an object
has a definite owner and a definite,
predictable lifespan.

My Question: To what extent does RAII substitute other design patterns like Garbage Collection? I am assuming that manual memory management is not used to represent shared ownership in the system.

Answer 1

To what extent does RAII substitute other design patterns like Garbage Collection? I am assuming that manual memory management is not used to represent shared ownership in the system

Hmm, with GC, you don’t really have to think about ownership. The object stays around as long as anyone needs it. Shared ownership is the default and the only choice.

And of course, everything can be done with shared ownership. But it sometimes leads to very clumsy code, because you can’t control or limit the lifetime of an object. You have to use C#’s using blocks, or try/finally with close/dispose calls in the finally clause to ensure that the object gets cleaned up when it goes out of scope.

In those cases, RAII is a much better fit: When the object goes out of scope, all the cleanup should happen automatically.

RAII replaces GC to a large extent. 99% of the time, shared ownership isn’t really what you want ideally. It is an acceptable compromise, in exchange for saving a lot of headaches by getting a garbage collector, but it doesn’t really match what you want. You want the resource to die at some point. Not before, and not after. When RAII is an option, it leads to more elegant, concise and robust code in those cases.

RAII is not perfect though. Mainly because it doesn’t deal that well with the occasional case where you just don’t know the lifetime of an object. It has to stay around for a long while, as long as anyone uses it. But you don’t want to keep it around forever (or as long as the scope surrounding all the clients, which might just be the entirety of the main function).

In those cases, C++ users have to “downgrade” to shared ownership semantics, usually implemented by reference-counting through shared_ptr. And in that case, a GC wins out. It can implement shared ownership much more robustly (able to handle cycles, for example), and more efficiently (the amortized cost of ref counting is huge compared to a decent GC)

Ideally, I’d like to see both in a language. Most of the time, I want RAII, but occasionally, I have a resource I’d just like to throw into the air and not worry about when or where it’s going to land, and just trust that it’ll get cleaned up when it’s safe to do so.