40.2. Selecting Data From a Ring Buffer

Many data consumers are not interested in all of the item types that could be put in a ring buffer. This section describes the infrastructure for selecting subsets of the data from ring buffers that are formatted as described in this chapter.

The ring buffer low level access library provides two powerful mechanisms for manipulating ring buffers based the concept of predicates. A predicate is a function that implement the operator() method in a way that it returns a bool. Predicate objects can therefore be thought of as functions that can have state that persists between calls.

CRingBuffer has two member functions blockWhile which periodically invokes a predicate until it returns false blocking for a settable time period between calls, and for a maximum timeout. While which simply repeatedly calls a predicate until it returns false. (like blockWhile but without blocking between calls to the predicate.

Armed with predicates, these two member functions an an understanding that items in the ring will have the item format described in the previous section, it is possible to implement a scheme for selectively obtaining data from a ring buffer.

The base class for this selectivity is CRingSelectionPredicate defined in CRingSelectionPredicate.h. It provides base classes with the capability of specifying a list of item types called a selection map. Elements of this map consist of an item type and a flag. The virtual function selectThis is given an item type peeked from the ring buffer and returns false if the item is acceptable. Unacceptable items are skipped, and true is returned so that While continues to loop.

If selectThis returns true, the framework insepcts the type's flag. If false, the predicate as a whole returns true. I false, the predicate skips the item if it is not the last one in the ring.

Two pre-packaged specific derivations of CRingSelectionPredicate have been built. In CDesiredTypesPredicate, any type not in the list is assumed to be undesirable and skipped. In CAllButPredicate, the list of types define exceptiosn to the rule that all items are desired without sampling. Items in the list with the sample flag false, are assumed to be undesirable and are skipped. items in the list with the sample flag true, are assumed to be wanted but only in sample mode.