Implementing the Operations

The second step in building an application is to write the code that implements the operations of its interfaces. Currently, this code must be written in C++. The Shore project intends to support other implementation languages in future releases. The implementation code for the search tree example is contained in four files, one for each interface: tree.C, word.C, cite.C, and document.C.

On the whole, this code is similar to the code one would write to implement the C++ classes generated from the interface definitions. However, there are several points to note:

• Each of these files #includes the header file stree.h, which is generated from the interface definitions. This file, in turn, #includes header files from the Shore library that define a variety of types and macros mentioned below.

• Types defined as ref<T> in SDL are written as Ref<T> in C++. Similar remarks apply to set, bag, etc.

• Operations declared const in SDL become const member functions in the C++ binding.

• Values of type Ref<T> can be used as if they were of type const T*. For example, consider the body of the function SearchTree::find. The attribute root of interface SearchTree, which is declared to have type ref<Word>, is translated to a data member of type Ref<Word> of class SearchTree. Thus the test "if (root)" checks whether the root pointer is null, and the call "root->find(str)" invokes the const member function Word::find.

• The special member function Ref<T>::update converts a value of type Ref<T> into a pointer of type T*. In other words, update explicitly performs the conversion from Ref to pointer described in the preceding paragraph, but without the const qualification. For example, consider the body of the function SearchTree::insert. Since the automatic conversion described in the preceding paragraph can only convert a Ref<Word> to a const Word *, the compiler would reject root->find_or_add(s) because Word::find_or_add is a non-const member function of Word. Thus we write root.update()->find_or_add(s) to explicitly convert root from Ref<Word> to Word *. The update function has the runtime effect of locking the referenced object to prevent interference from other users and marking it as (potentially) modified, so that it will be written back to persistent storage at the end of a transaction.