The With statement

13.2.8 The With statement

The with statement serves to access the elements of a record or object or class, without having to specify the element’s name each time. The syntax for a with statement is

_________________________________________________________________________________________________________
With statement

-- -|- --- - -------------------
with statement |-variable-reference-| do statement
,
___________________________________________________________________

The variable reference must be a variable of a record, object or class type. In the with statement, any variable reference, or method reference is checked to see if it is a field or method of the record or object or class. If so, then that field is accessed, or that method is called. Given the declaration:

Type
  Passenger = Record
    Name : String[30];
    Flight : String[10];
  end;

Var
  TheCustomer : Passenger;

The following statements are completely equivalent:

TheCustomer.Name := 'Michael';
TheCustomer.Flight := 'PS901';

and

With TheCustomer do
  begin
  Name := 'Michael';
  Flight := 'PS901';
  end;

The statement

With A,B,C,D do Statement;

is equivalent to

With A do
With B do
With C do
With D do Statement;

This also is a clear example of the fact that the variables are tried last to first, i. e., when the compiler encounters a variable reference, it will first check if it is a field or method of the last variable. If not, then it will check the last-but-one, and so on. The following example shows this;

Program testw;
Type AR = record
      X,Y : Longint;
     end;
     PAR = ^Ar;

Var S,T : Ar;
begin
  S.X := 1;S.Y := 1;
  T.X := 2;T.Y := 2;
  With S,T do
    WriteLn (X,' ',Y);
end.

The output of this program is

2 2

Showing thus that the X,Y in the WriteLn statement match the T record variable.

Remark When using a With statement with a pointer, or a class, it is not permitted to change the pointer or the class in the With block. With the definitions of the previous example, the following illustrates what it is about:

Var p : PAR;

begin
  With P^ do
   begin
   // Do some operations
   P:=OtherP;
   X:=0.0;  // Wrong X will be used !!
   end;

The reason the pointer cannot be changed is that the address is stored by the compiler in a temporary register. Changing the pointer won’t change the temporary address. The same is true for classes.

[next] [prev] [prev-tail] [front] [up]