READONLY Parameters

A READONLY parameter is similar to a value parameter in that the routine cannot directly modify its actual parameter, which can be a constant, an expression, or a variable. READONLY differs from a value parameter in that the routine cannot modify the formal parameter: you cannot assign a value to the formal READONLY parameter, pass it to a VAR or ANYVAR parameter, or pass it to either of the predefined functions addr, baddress, or waddress.

A READONLY parameter is similar to a VAR or ANYVAR parameter in that its actual parameter is passed by reference. If the actual parameter is an expression or constant, a copy of its value is passed by reference.

Example

PROGRAM prog;
$STANDARD_LEVEL 'HP_MODCAL'$
 
TYPE
   arraytype = ARRAY [1..10] OF integer;
 
CONST
   arrayconst = arraytype [10 OF 0];
 
VAR
   arrayvar : arraytype;
 
FUNCTION arrayfunc : arraytype; EXTERNAL;
 
PROCEDURE p (            valuep : arraytype;
                       VAR varp : arraytype;
             READONLY readonlyp : arraytype); EXTERNAL;
 
BEGIN
   p(arrayconst,   {value is passed}
     arrayconst,   {illegal  must be a variable}
     arrayconst);  {address of copy of value is passed}
 
   p(arrayvar,     {value is passed}
     arrayvar,     {address is passed}
     arrayvar);    {address is passed}
 
   p(arrayfunc,    {value is passed}
     arrayfunc,    {illegal  must be a variable}
     arrayfunc);   {address of copy of value is passed}
END.

The comments in the preceding program explain the differences in passing a constant (arrayconst), a variable (arrayvar), and an expression (a call to the function arrayfunc) to a value parameter (valuep), a VAR parameter (varp), and a READONLY parameter (readonlyp).