STXXL  1.4.1
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Reference Counted (Shared) Objects

Some objects in STXXL are reference counted. This is usually done for large objects, which should not be copied deeply in assignments. Instead, a reference counting pointer is used, which holds a handle while the pointer exists and deletes the object once all pointers go out of scope. Examples are matrices and stream::sorted_runs.

The method used is called counting_ptr or intrusive reference counting. This is similar, but not identical to boost or TR1's shared_ptr.

The counting_ptr is accompanied by counted_object, which contains the actual reference counter (a single integer). A reference counted object must derive from counted_object :

struct something : public stxxl::counted_object

Code that now wishes to use pointers referencing this object, will typedef an counting_ptr, which is used to increment and decrement the included reference counter automatically.

typedef stxxl::counting_ptr<something> something_ptr
// create new instance of something
something_ptr p1 = new something;
// create a new reference to the same instance (no deep copy!)
something_ptr p2 = p1;
// this block end will decrement the reference count, but not delete the object
// this block end will delete the object

The counting_ptr can generally be used like a usual pointer or shared_ptr (see the docs for more).

There is also const_counting_ptr to return const objects (note that a const counting_ptr will only render the pointer const, not the enclosed object).