C++ – Smart pointers: who owns the object?

c++memory-managementownership-semanticssmart-pointers

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.

Best Solution

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.