Nested Invocation

The allowance of nested invocation is what makes structured programming languages (like Pascal, C and others) powerful. You can invoke a subroutine from a subroutine. There is no theoretical limitation as to how many levels of invocations a program can nest.

Parameter passing in nested invocation requires some additional explanation. This is especially the case with passing by reference.

Let us consider the following program:

var gx, gy : integer;

procedure sub1(x1 : integer; var y1 : integer);
  begin
    y1 := y1 + x1
  end;
 
procedure sub2(var x2 : integer; var y2 : integer);
  begin
    x2 := x2 * 2;
    sub1(x2, y2)
  end;

begin
  gx := 6;
  gy := 11;
  sub2(gx, gy)
end.

Let's see what happens when this program executes:

• gx and gy are initialized to 6 and 11, respectively.
• sub2 is invoked, x2 becomes a reference to gx, y2 becomes a reference to gy.
• x2 is doubled. Since x2 is a reference to gx, the value of gx is doubled from 6 to 12.
• sub1 is invoked, the value of x2 is passed by value of x1 of sub1, which means x1 has a copy of the value of x2. What is x2? x2 is a reference to gx. As a result, formal parameter x1 of sub1 has a value of 12. y2 is passed by reference to y1. y2 is a reference to gy. This means y1 is also a reference to gy.
• we add x1 to y1. x1 has a value of 12. y1 is a reference to gy, which has a value of 11. The sum is 23, and it is stored to y1. Since y1 is a reference to gy, 23 is actually store to gy!
• we now return from sub1.
• we now return from sub2.
• in the main program gx has a value of 12, and gy has a value of 23.

What if we change formal parameter y2 from passed-by-reference to passed-by-value? It becomes a snapshot of the value of gy, but changes to y2 is no longer reflected in gy. When we pass y2 by reference to y1 of sub1, we make y1 a reference to y2 instead of gy. This means the outcome of the program is that gx still has a value of 12, but gy has a value of 11 instead of 23 because changes to y1 in sub1 changes y2, which is only a copy of gy, leaving gy unmodified.