4.5 Properties

A global block can declare properties, just as they could be dened in a class. The dierence is that the global property does not need a class instance: there is only 1 instance of this property. Other than that, a global property behaves like a class property. The read/write speciers for the global property must also be regular procedures, not methods. The concept of a global property is specic to Free Pascal, and does not exist in Delphi.

The concept of a global property can be used to 'hide' the location of the value, or to calculate the value on the y, or to check the values which are written to the property.

The declaration is as follows:

_________________________________________________________________________________________________________ Properties
-- -- - ------------------- ------
property de nition identi er -property interface| property speci ers

-- --property interface -----------------------:-type identi er-
- -------------------property parameter list---------------------------
- - --|
index integerconstant

-- --property parameter list [--6parameter declaration--] ------------------
---------;---------

-- --property speci ers-|-------------|--------------|----------------
-read speci er- -write speci er -default speci er

-- --read speci er read - eld or function-------------------------------

-- --write speci er-write eld or procedure-----------------------------

-- -- --- ------------------------------------------
default speci er|default -constant-| |
------nodefault ------|

-- -- ---- ----------------------------------
eld or procedure - eld identi er-|
procedure identi er

-- -- eld or function---| eld identi er---------------------------------
-function identi er--
___________________________________________________________________

The following is an example:

{$mode objfpc}
unit testprop;

Interface

Function GetMyInt : Integer;
Procedure SetMyInt(Value : Integer);

Property
  MyProp : Integer Read GetMyInt Write SetMyInt;

Implementation

Uses sysutils;

Var
  FMyInt : Integer;

Function GetMyInt : Integer;

begin
  Result:=FMyInt;
end;

Procedure SetMyInt(Value : Integer);

begin
  If ((Value mod 2)=1) then
    Raise Exception.Create('MyProp can only contain even value');
  FMyInt:=Value;
end;

end.

The read/write speciers can be hidden by declaring them in another unit which must be in the uses clause of the unit. This can be used to hide the read/write access speciers for programmers, just as if they were in a private section of a class (discussed below). For the previous example, this could look as follows:

{$mode objfpc}
unit testrw;

Interface

Function GetMyInt : Integer;
Procedure SetMyInt(Value : Integer);

Implementation

Uses sysutils;

Var
  FMyInt : Integer;

Function GetMyInt : Integer;

begin
  Result:=FMyInt;
end;

Procedure SetMyInt(Value : Integer);

begin
  If ((Value mod 2)=1) then
    Raise Exception.Create('Only even values are allowed');
  FMyInt:=Value;
end;

end.

The unit testprop would then look like:

{$mode objfpc}
unit testprop;

Interface

uses testrw;

Property
MyProp : Integer Read GetMyInt Write SetMyInt;

Implementation

end.

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