RE: Not grasping concept

["Andy Holmer" <aholmer@xxxxxxxxxx>]

Tim,
 

        
        I am having a VERY difficult time grasping the concepts behind
        prototypes/return values/procedure interfaces.
        Why is this necessary?
        
         
        The prototype being copied in after the procedure is another way
to make sure that your parameters will line up.  It would do no good to
make sure that the procedure is called the same way (via /Copy member ),
if the procedure was not interfaced the same way it is prototyped.  By
checking the /Copy member at compile time for the procedure, it helps
insure that the parameter list you compiled with, and the parameter list
you call with, are actually the same.
         

        Didn't the included prototype already declare these fields?
        Wasn't that the purpose of the prototype?
        
                // Declare the field to be returned:
                D Footage                     15S 3
        Why is this necessary?
         
        You do not actually need to declare a work variable here.
Operations can be done on the return line and it does return the
variable type you defined.

        Didn't the prototype already declare the return value
        type/length/decimals?
        
        Yes, it did.  Again operation can be done on the return line.

        I must be missing some very basic concepts, because I would have
thought
        that the following was ALL that SHOULD be necessary for
subprocedure
        TestSP:
        
                H NOMAIN
                // Include the prototype
                /Copy PRODLIB/QRPGSRC/PROTSP
                // Calculate and return value
                /Free
                 TestSP = Unit * Qnty * Long;
                /End-Free
        
        You could code this Return (Unit * Qty * Long);
        
        In the world of C and C++ that I have been introduced to, it has
always been necessary to define the interface and the implementation.
Including the interface with the implementtion insures that in the /Copy
member has not been changed, you can be sure that your procedure will
pass its parameters correctly.  Last, but not least, the P-Specs show
the compiler where the procedure begins and ends.  There can be several
return statements in a proceudure and the P-Specs keep the procedures
separated.
         
        HTH,
        Andy