Interface vs Implementation

The interface to a class of objects contains the information that a client must know to use an object-namely, the names of its attributes and the signatures of its methods. The interface is described in a formal language independent of the programming language used to implement the object's methods. In SOM, the formal language used to define object interfaces is the Interface Definition Language (IDL), standardized by CORBA.

The implementation of a class of objects (that is, the procedures that implement methods) is written in the implementer's preferred programming language. This language can be object-oriented (for instance, C++) or procedural (for instance, C).

A completely implemented class definition, then, consists of two main files:

• An IDL specification of the interface to instances of the class (the interface definition file (or .idl file) and

• Method procedures written in the implementer's language of choice (the implementation file).

The interface definition file has an .idl extension, as noted. The implementation file, however, has an extension specific to the language in which it is written. For example, implementations written in C have a .c extension, and implementations written in C++ have a .C (for AIX) or .cpp (for OS/2) extension.

To assist users in implementing SOM classes, the SOMobjects Toolkit provides a SOM Compiler. The SOM Compiler takes as input an object interface definition file (the .idl file) and produces a set of binding files that make it convenient to implement and use a SOM class whose instances are objects that support the defined interface. The binding files and their purposes are as follows:

• An implementation template that serves as a guide for how the implementation file for the class should look. The class implementer fills in this template file with language-specific code to implement the methods that are available on the class' instances.

• Header files to be included (a) in the class's implementation file and (b) in client programs that use the class.

These binding files produced by the SOM Compiler bridge the gap between SOM and the object model used in object-oriented languages (such as C++), and they allow SOM to be used with non-object-oriented languages (such as C). The SOM Compiler currently produces binding files for the C and C++ programming languages. SOM can also be used with other programming languages; the bindings simply offer a more convenient programmer's interface to SOM. Vendors of other languages may also offer SOM bindings; check with your language vendor for possible SOM support.

The subsequent sections of this chapter provide full syntax for SOM IDL and the SOM Compiler.