OO COBOL: Compiling and Linking

In short: it's a pain.

It took me only a couple of hours to get my first itty bitty OO programs to compile -- one class definition and one program to drive it. It took me about three days to get them to link and run.

Building an OO program involves far more than the usual compile-link-run process. I haven't figured it all out yet, but the major pieces seem to be as follows:

For a class definition, the COBOL compiler writes a file in Interface Definition Language (IDL), normally a member in an IDL library. IDL is a specialized language, similar to C, for specifying class interfaces. A DD statement in the compile step tells the compiler where to put the IDL file.

In a subsequent step, the SOM compiler compiles the IDL into machine-readable gibberish, which it stores in an Interface Repository (IR). There are also options whereby the SOM compiler can create header files and program skeletons for C or C++ programs. If you're ambitious, you can even coax the SOM compiler into writing something completely different, such as documentation or ADA.

There's a generic IR for built-in SOM classes. You're expected to create one or more levels of separate IRs by installation, application, and the like.

Once the IR contains a representation of the class interface, the COBOL compiler can consult it to verify that it is invoking the methods with the right parameters.

The compile step requires a DD statement for a profile which defines a series of environmental variables. Among other things, these variables tell the compiler which repositories to consult.

The compiler puts the object code into an object library in the usual way. However, in order to support names longer than eight characters, you need to keep a special member in the library to map long names to mangled eight-character names. You have to know how to invoke the tools which maintain this mapping.

Different kinds of compiles need different kinds of JCL. Compiling a class definition is different from compiling a client program, and each is different from compiling a program which doesn't use objects. You need to follow yet another procedure if two or more classes refer to each other.

If you are adopting OO COBOL for the first time without an experienced hand to guide you, the complexity of these procedures will be a substantial obstacle.

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