8.7.7 Class operators

Class operators are slightly dierent from the operators above in the sense that they can only be used in class expressions which return a class. There are only 2 class operators, as can be seen in table (8.8).

Table 8.8:

Class operators

Operator	Action
is	Checks class type
as	Conditional typecast

An expression containing the is operator results in a boolean type. The is operator can only be used with a class reference or a class instance. The usage of this operator is as follows:

Object is Class

This expression is completely equivalent to

Object.InheritsFrom(Class)

If Object is Nil, False will be returned.

The following are examples:

Var
  A : TObject;
  B : TClass;

begin
  if A is TComponent then ;
  If A is B then;
end;

The as operator performs a conditional typecast. It results in an expression that has the type of the class:

Object as Class

This is equivalent to the following statements:

  If Object=Nil then
    Result:=Nil
  else if Object is Class then
    Result:=Class(Object)
  else
    Raise Exception.Create(SErrInvalidTypeCast);

Note that if the object is nil, the as operator does not generate an exception.

The following are some examples of the use of the as operator:

Var
  C : TComponent;
  O : TObject;

begin
  (C as TEdit).Text:='Some text';
  C:=O as TComponent;
end;

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Found a problem.

The following works fine:

test10a.pp
----------------------------------------------------------------------
unit test10a;

interface

const
a = 'a';
b = 'b';

implementation

begin
end.
----------------------------------------------------------------------

test10b.pp
----------------------------------------------------------------------
program test;

uses
test10a;

var
c : Char;

begin
Write( 'A, or B: ' );
Readln( c );
case c of
a ,
b : Writeln( 'ok: ' + c );
else
Writeln( 'oops ' + c );
end;
end.
----------------------------------------------------------------------

This, however, fails.

test11a.pp
----------------------------------------------------------------------
unit test11a;

interface

const
a : char = 'a';
b : char = 'b';

implementation

begin
end.
----------------------------------------------------------------------

test11b.pp
----------------------------------------------------------------------
program test11b;

uses
test11a;

var
c : Char;

begin
Write( 'A, or B: ' );
Readln( c );
case c of
a ,
b : Writeln( 'ok: ' + c );
else
Writeln( 'oops ' + c );
end;
end.
----------------------------------------------------------------------

The only difference being the type identification of the char constants.

Bob

-- Bob Gibson on December 21, 2006 01:54 AM (view details)

simplier failure

program test12;

const
a : char = 'a';
b : char = 'b';

var
c : Char;

begin
Write( 'A, or B: ' );
Readln( c );
case c of
a ,
b : Writeln( 'ok: ' + c );
else
Writeln( 'oops ' + c );
end;
end.

-- Bob Gibson on December 21, 2006 02:01 AM (view details)

Statement-List in "else" part

-- Unregistered Visitor on April 22, 2007 06:10 PM (view details)