The standard Theta library comprises 16 classes and 19 abstract types, and several other small libraries are under development. Considering the two largest libraries, one implementing the OO7 database benchmark [CDN93], and another implementing the Labbase database [RSG95], there are a total of 51 abstract types and 45 classes. Table 1 shows the number of words of per-object dispatch overhead incurred by these classes with the bidirectional layout and with a standard C++ layout. Each entry in the table shows the number of classes that have a particular number of words of dispatch overhead. Of these classes, only one requires more than a single dispatch vector with the bidirectional layout. It occupies a position in the hierarchy very similar to C3 in Figure 11, and has two dispatch vectors.
The per-object space overhead is clearly lower for the bidirectional layout. Per-class space overhead is also less: the bidirectional layouts have a total of 46 dispatch vectors, compared to 73 in the C++ layout. The bidirectional dispatch vectors are larger on the average than the C++ dispatch vectors, but they do not repeat any information that is not also repeated by the C++ dispatch vectors. Since only one trampoline routine needs to be generated for the bidirectional layout, total space usage is smaller. However, per-class space usage is less important than per-object space usage, since the total number of objects in any significant program is likely to be much larger than the total number of classes in use.