the problem with passing a const reference

["Steve Richter" <srichter@xxxxxxxxxxxxx>]

There is a problem with passing by const reference that I did not see
mentioned in the recent discussion.

you have a qualified data struct:

d  Cust          ds           qualified
d   Nbr                    6a
d   Name                  30a

and a proc that reads the OrCust database by key and returns a Cust ds:
p  Read_Cust     b
d  Read_Cust     pi
d   InNbr                  6a const
d   OutCust                   likeds(Cust)
 /free
      clear      OutCust ;
      open       OrCust1 ;
      chain      InNbr OrCust1r ;
      close      OrCust1 ;
      if         %found( ) ;
        OutCust  = rCust ;  // trust me.  all the OrCust1 flds are mapped
                            // into a data struct named "rCust"
      endif ;
 /end-free
p                e

** Now this proc is used in code:
d MyCust         ds           likeds(Cust)
 /free
      MyCust.Nbr = 'abc' ;
      Read_Cust( MyCust.Nbr: MyCust ) ;  // will never work like this!!
 /end-free

when the Read_Cust proc is used like so:
  Read_Cust( MyCust.Nbr: MyCust ) ;
the proc call will never work.

when the Read_Cust proc clears parm2, "OutCust", it clears the "MyCust" ds
that was passed by reference to the proc.  Since the input key fld is a
member of that ds, the key is cleared also!

If parm1 of the Read_Cust parm is declared passed by "value", then the proc
does work.

There are other good reasons for passing by value instead of const
reference. Separating the call stack entries from each other as much as
possible is good programming practice.

Passing by const reference should be used with caution and if there is no
performance gain, should not be used.

-Steve Richter