Quick start manual

Procedures and functions 6-13
Parameters
These functions return the same result, but only the second one—DoubleByRef—can
change the value of a variable passed to it. Suppose we call the functions like this:
var
I, J, V, W: Integer;
begin
I := 4;
V := 4;
J := DoubleByValue(I); // J = 8, I = 4
W := DoubleByRef(V); // W = 8, V = 8
end;
After this code executes, the variable I, which was passed to DoubleByValue, has the
same value we initially assigned to it. But the variable V, which was passed to
DoubleByRef, has a different value.
A value parameter acts like a local variable that gets initialized to the value passed in
the procedure or function call. If you pass a variable as a value parameter, the
procedure or function creates a copy of it; changes made to the copy have no effect on
the original variable and are lost when program execution returns to the caller.
A variable parameter, on the other hand, acts like a pointer rather than a copy.
Changes made to the parameter within the body of a function or procedure persist
after program execution returns to the caller and the parameter name itself has gone
out of scope.
Even if the same variable is passed in two or more var parameters, no copies are
made. This is illustrated in the following example.
procedure AddOne(var X, Y: Integer);
begin
X := X + 1;
Y := Y + 1;
end;
var I: Integer;
begin
I := 1;
AddOne(I, I);
end;
After this code executes, the value of I is 3.
If a routine’s declaration specifies a var parameter, you must pass an assignable
expression—that is, a variable, typed constant (in the {$J+} state), dereferenced
pointer, field, or indexed variable—to the routine when you call it. To use our
previous examples, DoubleByRef(7) produces an error, although DoubleByValue(7) is
legal.
Indexes and pointer dereferences passed in var parameters—for example,
DoubleByRef(MyArray[I])—are evaluated once, before execution of the routine.
Constant parameters
A constant (const) parameter is like a local constant or read-only variable. Constant
parameters are similar to value parameters, except that you can’t assign a value to a
constant parameter within the body of a procedure or function, nor can you pass one