© 2006 John Abbott
GNU Free Documentation License, Version 1.2

CoCoALib Documentation Index

User documentation for files SmartPtrIRC

The name SmartPtrIRC stands for Smart Pointer with Intrusive Reference Count. The desired behaviour is achieved through two cooperating classes: SmartPtrIRC and IntrusiveReferenceCount. These classes exist to facilitate implementation of smart pointers with reference counting. The suggested use is as follows. Make your implementation class inherit protected-ly from IntrusiveReferenceCount, and in your implementation class declare the class SmartPtrIRC<MyClass> as a friend. You can now use the class SmartPtrIRC<MyClass> as a reference counting smart pointer to your class.

The template argument of the class SmartPtrIRC specifies the type of object pointed to; if you want the objects pointed at to be const then put the keyword "const" in the template argument like this SmartPtrIRC<const MyClass>. Creating a new SmartPtrIRC to a datum will increment its reference count; conversely, destroying the SmartPtrIRC decrements the ref count (and destroys the object pointed at if the ref count reaches zero, see IntrusiveReferenceCount::myRefCountDec). Five operations are available for SmartPtrIRC values:

let SPtr be a SmartPtrIRC value

The class IntrusiveReferenceCount is intended to be used solely as a base class. Note the existence of IntrusiveReferenceCount::myRefCountZero which forces the reference count to be zero. For instance, this is used in ring implementations where the ring object contains some circular references to itself; after creating the circular references the ring constructor then resets the reference count to zero so that the ring is destroyed at the right moment. SEE BUGS SECTION.

IMPORTANT NOTE: it is highly advisable to have myRefCountZero() as the very last operation in every contructor of a class derived from IntrusiveReferenceCount, i.e. intended to be used with SmartPtrIRC.

Maintainer documentation for files SmartPtrIRC

The entire implementation is in the .H file: a template class, and another class with only inline member functions. Inlining is appropriate as the functions are extremely simple and we expect them to be called a very large number of times.

The implementation is quite straightforward with one important detail: the destructor of IntrusiveReferenceCount must be virtual because myRefCountDec does a polymorphic delete through a pointer to IntrusiveReferenceCount when the count drops to zero. The book by Sutter and Alexandrescu gives wrong advice (in article 50) about when to make destructors virtual!

The fn mySwap is a member fn because I couldn't figure out how to make it a normal (templated?) function. I also feared there might have been some problems with the template fn std::swap.

Bugs, Shortcomings and other ideas

Should myRefCountZero be eliminated? It is not strictly necessary (just call myRefCountDec after each operation which incremented the ref count. This is related to how rings create their zero and one elements (and possibly other elements which should always exist, e.g. indets in a poly ring).

Could ref count overflow? Perhaps size_t is always big enough to avoid overflow?

It may be possible to replace all this code with equivalent code from the BOOST library. But so far (Nov 2006) the shared_ptr implementation in BOOST is not documented, so presumably should not be used. As there is no documentation I have not verified the existence of a set ref count to zero function; I rather suspect that it does not exist.