Smart pointers: who owns the object?

C++ is all about memory ownership - aka ownership semantics.

It is the responsibility of the owner of a chunk of dynamically allocated memory to release that memory. So the question really becomes who owns the memory.

In C++ ownership is documented by the type a raw pointer is wrapped inside thus in a good (IMO) C++ program it is very rare (rare, not never) to see raw pointers passed around (as raw pointers have no inferred ownership thus we can not tell who owns the memory and thus without careful reading of the documentation you can't tell who is responsible for ownership).

Conversely, it is rare to see raw pointers stored in a class each raw pointer is stored within its own smart pointer wrapper. (N.B.: If you don't own an object you should not be storing it because you can not know when it will go out of scope and be destroyed.)

So the question:

• What type of ownership semantic have people come across?
• What standard classes are used to implement those semantics?
• In what situations do you find them useful?

Lets keep 1 type of semantic ownership per answer so they can be voted up and down individually.

Summary:

Conceptually, smart pointers are simple and a naive implementation is easy. I have seen many attempted implementations, but invariably they are broken in some way that is not obvious to casual use and examples. Thus I recommend always using well tested smart pointers from a library rather than rolling your own. std::auto_ptr or one of the Boost smart pointers seem to cover all my needs.

std::auto_ptr<T>:

Single person owns the object. Transfer of ownership is allowed.

Usage: This allows you to define interfaces that show the explicit transfer of ownership.

boost::scoped_ptr<T>

Single person owns the object. Transfer of ownership is NOT allowed.

Usage: Used to show explicit ownership. Object will be destroyed by destructor or when explicitly reset.

boost::shared_ptr<T> (std::tr1::shared_ptr<T>)

Multiple ownership. This is a simple reference counted pointer. When the reference count reaches zero, the object is destroyed.

Usage: When an object can have multiple owers with a lifetime that can not be determined at compile time.

boost::weak_ptr<T>:

Used with shared_ptr<T> in situations where a cycle of pointers may happen.

Usage: Used to stop cycles from retaining objects when only the cycle is maintaining a shared refcount.

Simple C++ Model

In most modules I saw, by default, it was assumed that receiving pointers was not receiving ownership. In fact, functions/methods abandoning ownership of a pointer were both very rare and explicitly expressed that fact in their documentation.

This model assumes that the user is owner only of what he/she explicitly allocates. Everything else is automatically disposed of (at scope exit, or through RAII). This is a C-like model, extended by the fact most pointers are owned by objects that will deallocate them automatically or when needed (at said objects destruction, mostly), and that the life duration of objects are predictable (RAII is your friend, again).

In this model, raw pointers are freely circulating and mostly not dangerous (but if the developer is smart enough, he/she will use references instead whenever possible).

• raw pointers
• std::auto_ptr
• boost::scoped_ptr

Smart Pointed C++ Model

In a code full of smart pointers, the user can hope to ignore the lifetime of objects. The owner is never the user code: It is the smart pointer itself (RAII, again). The problem is that circular references mixed with reference counted smart pointers can be deadly, so you have to deal both with both shared pointers and weak pointers. So you have still ownership to consider (the weak pointer could well point to nothing, even if its advantage over raw pointer is that it can tell you so).

• boost::shared_ptr
• boost::weak_ptr

Conclusion

No matter the models I describe, unless exception, receiving a pointer is not receiving its ownership and it is still very important to know who owns who. Even for C++ code heavily using references and/or smart pointers.

For me, these 3 kinds cover most of my needs:

shared_ptr - reference-counted, deallocation when the counter reaches zero

weak_ptr - same as above, but it's a 'slave' for a shared_ptr, can't deallocate

auto_ptr - when the creation and deallocation happen inside the same function, or when the object has to be considered one-owner-only ever. When you assign one pointer to another, the second 'steals' the object from the first.

I have my own implementation for these, but they are also available in Boost.

I still pass objects by reference (const whenever possible), in this case the called method must assume the object is alive only during the time of call.

There's another kind of pointer that I use that I call hub_ptr. It's when you have an object that must be accessible from objects nested in it (usually as a virtual base class). This could be solved by passing a weak_ptr to them, but it doesn't have a shared_ptr to itself. As it knows these objects wouldn't live longer than him, it passes a hub_ptr to them (it's just a template wrapper to a regular pointer).

Don't have shared ownership. If you do, make sure it's only with code you don't control.

That solves 100% of the problems, since it forces you to understand how everything interacts.

• Shared Ownership
• boost::shared_ptr

When a resource is shared between multiple objects. The boost shared_ptr uses reference counting to make sure the resource is de-allocated when everybody is finsihed.

std::tr1::shared_ptr<Blah> is quite often your best bet.

From boost, there's also the pointer container library. These are a bit more efficient and easier to use than a standard container of smart pointers, if you'll only be using the objects in the context of their container.

On Windows, there are the COM pointers (IUnknown, IDispatch, and friends), and various smart pointers for handling them (e.g. the ATL's CComPtr and the smart pointers auto-generated by the "import" statement in Visual Studio based on the _com_ptr class).

• One Owner
• boost::scoped_ptr

When you need to allocate memory dynamically but want to be sure it gets deallocated on every exit point of the block.

I find this usefull as it can easily be reseated, and released without ever having to worry about a leak

I don't think I ever was in a position to have shared ownership in my design. In fact, from the top of my head the only valid case I can think of is Flyweight pattern.

yasper::ptr is a lightweight, boost::shared_ptr like alternative. It works well in my (for now) small project.

In the web page at http://yasper.sourceforge.net/ it's described as follows:

> Why write another C++ smart pointer? > There already exist several high > quality smart pointer implementations > for C++, most prominently the Boost > pointer pantheon and Loki's SmartPtr. > For a good comparison of smart pointer > implementations and when their use is > appropriate please read Herb Sutter's > The New C++: Smart(er) Pointers. In > contrast with the expansive features > of other libraries, Yasper is a > narrowly focused reference counting > pointer. It corresponds closely with > Boost's shared_ptr and Loki's > RefCounted/AllowConversion policies. > Yasper allows C++ programmers to > forget about memory management without > introducing the Boost's large > dependencies or having to learn about > Loki's complicated policy templates. > Philosophy > > * small (contained in single header) > * simple (nothing fancy in the code, easy to understand) > * maximum compatibility (drop in replacement for dumb pointers) > > The last point can be dangerous, since > yasper permits risky (yet useful) > actions (such as assignment to raw > pointers and manual release) > disallowed by other implementations. > Be careful, only use those features if > you know what you're doing!

There is another frequently used form of single-transferable-owner, and it is preferable to auto_ptr because it avoids the problems caused by auto_ptr's insane corruption of assignment semantics.

I speak of none other than swap. Any type with a suitable swap function can be conceived of as a smart reference to some content, which it owns until such time as ownership is transferred to another instance of the same type, by swapping them. Each instance retains its identity but gets bound to new content. It's like a safely rebindable reference.

(It's a smart reference rather than a smart pointer because you don't have to explicitly dereference it to get at the content.)

This means that auto_ptr becomes less necessary - it's only needed to fill the gaps where types don't have a good swap function. But all std containers do.

• One Owner: Aka delete on Copy
• std::auto_ptr

When the creator of the object wants to explicitly hand ownership to somebody else. This is also a way documenting in the code I am giving this to you and I am no longer tracking it so make sure you delete it when you are finished.