The interface to a class of objects contains the information that a client must know to use an object - namely, the signatures of its methods and the names of its attributes. 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 (described in Chapter 4, "SOM IDL and the SOM Compiler").
The implementation of a class of objects (that is, the procedures that implement the methods and the instance variables that store an object's state) is written in the implementor'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:
The SOM Compiler provides the link between those two files: To assist users in implementing classes, the SOM Compiler produces a template implementation file - a type-correct guide for how the implementation of a class should look. Then, the class implementor modifies this template file to fully implement the class's methods. That process is the subject of the remainder of this chapter.
The SOM Compiler can also update the implementation file to reflect changes subsequently made to a class's interface definition file (the .idl file). These incremental updates include adding new stub procedures, adding comments, and changing method prototypes to reflect changes made to the method definitions in the IDL specification. These updates to the implementation file, however, do not disturb existing code in the method procedures. These updates are discussed further in "Running incremental updates of the implementation template file" later in this section.
For C programmers, the SOM Compiler generates a <filestem>.c file. For C++ programmers, the SOM Compiler generates a <filestem>.C file (for AIX) or a <filestem>.cpp file (for OS/2). To specify whether the SOM Compiler should generate a C or C++ implementation template, set the value of the SMEMIT environment variable, or use the -s option when running the SOM Compiler. (See "The SOM Compiler" in Chapter 4, "SOM IDL and the SOM Compiler.")
Note: As this chapter describes, a SOM class can be implemented by using C++ to define the instance variables introduced by the class and to define the procedures that implement the overridden and introduced methods of the class. Be aware, however, that the C++ class defined by the C++ usage bindings for a SOM class (described in Chapter 3) cannot be subclassed in C++ to create new C++ or SOM classes. [The reason why the C++ implementation of a SOM class involves the definition of C++ procedures (not C++ methods) to support SOM methods is that there is no language-neutral way to call a C++ method. Only C++ code can call C++ methods, and this calling code must be generated by the same compiler that generates the method code. In contrast, the method procedures that implement SOM methods must be callable from any language, without knowledge on the part of the object client as to which language is used to implement the resolved method procedure]