Implementation Methods

Implementation Methods:

Pass-By-Value (In mode)

     Actual parameter (argument) is used to initialize the formal parameter (parameter).
     Implemented by actual data transfer (physically copy)
     It takes a lot of space.

Pass-By-Result (Out mode)

Value determined in subprogram is returned to actual parameter.
Troubles (page 340)

There can be an actual parameter collision:

In function call: sub(p1, p1)
In function definition: there are two different parameter's names, void sub(int s1, int s2)

s1 --> p1
s2 --> p1

One of them will be overwritten.

Pass-By-Value-Result (In-Out mode)

It is a combination of above two.

Pass-By-Reference (In-Out mode)

     No duplicate space
     No copying
     Access to formal parameters is slow
     If only one-way communication is needed, this is not safe
     Allow for creation of aliases

Pass-By-Name (In-Out mode)

     The actual parameter is textually substituted for corresponding formal parameter in all occurrences in the subprogram.
     If the actual parameter is a constant, then this is equivalent to Pass-By-Value
     If the actual parameter is a scalar, then this is equivalent to Pass-By-Reference.

     Other methods: formal parameters are bound to actual values or addresses at the time of the subprogram call.
     This method: formal parameter is bound to an access method at the time of the subprogram call,
                         but the actual binding to a value or an address is delayed until the formal parameter is assigned or referenced.

See the following example from page 364 of second edition:
　

procedure BIGSUB;
   integer GLOBAL;
   integer array LIST[1:2];
   procedure SUB(PARAM);
      integer PARAM;
      begin
      PARAM := 3;
      GLOBAL := GLOBAL + 1;
      PARAM := 5
      end;
   begin
   LIST[1] := 2;
   LIST[2] := 2;
   GLOBAL := 1;
   SUB(LIST[GLOBAL])
   end;



After execution, the array LIST has the values 3 and 5.

It is used at compile time by macros in assembly languages and the generic parameters of the generic subprograms in C++, Java 5.0, and C# 2005

procedure BIGSUB; integer GLOBAL; integer array LIST[1:2]; procedure SUB(PARAM); integer PARAM; begin PARAM := 3; GLOBAL := GLOBAL + 1; PARAM := 5 end; begin LIST[1] := 2; LIST[2] := 2; GLOBAL := 1; SUB(LIST[GLOBAL]) end;