Class initialization

Class Initialization

Some classes that have class variables require initialization when the program is run. For example, class ReadFileStream needs to have its class variable stdinStream set to the application’s stdin file stream before the program can read from standard input. For this, you would use the library function xstdin, and use its value in stdinStreamVar.

__ctalkObjValPtr (stdinStreamVar, xstdin ());

Classes that require class variable initialization need to define a method, classInit, which is called by a constructor when the first object is created. An example is the ReadFileStream method new from the previous section.

Here is the classInit method for class ReadFileStream.

ReadFileStream classMethod classInit (void) {
  "Initializes the classes' standard input stream.  This
  method is normally called by the ReadFileStream
  constructor.  The method needs to be called only once;
  on further calls to 'new', it is a no-op."
  OBJECT *classObject,
    *stdinStreamVar,
    *classInitVar;

  if (self classInitDone)
    return NULL;

  classObject = __ctalkGetClass ("ReadFileStream");
  stdinStreamVar = __ctalkFindClassVariable ("stdinStream", TRUE);
  __ctalkInstanceVarsFromClassObject (stdinStreamVar);
  
  __ctalkObjValPtr (stdinStreamVar, xstdin ());

  classInitVar = 
    __ctalkCreateObjectInit ("classInitDone", "Integer",
			     "Magnitude",
			     classObject -> scope, "1");
  __ctalkAddClassVariable (classObject, "classInitDone", classInitVar);

  return NULL;
}

A classInit method needs to be called only once. The method checks the classInitDone instance variable to determine the method has already performed the initialization that the class needs.