Property

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The reserved word property is part of object-oriented programming. It can allow various levels (read, read/write etc) outside access to variables inside a Class.

Example

type
TCar = class
private
FColor: string;
FBuildYear: integer;
procedure SetColor(CarColor: string);
public
property Color: string read FColor write SetColor; //Reads directly from the FColor variable;
// directs writes to the SetColor procedure that then changes the FColor variable.
// Another option could be to just do "write FColor" to directly write to the FColor variable..
end;

procedure TCar.SetColor(CarColor: string);
begin
//we can do other things here, too, such as housekeeping, notifying others of changes,
//validating against other variables, logging, etc -e.g.
if CarColor='Pink' then
ShowMessage('Note: pink is not a very common color for a car.');
FColor:=CarColor;
end;

procedure PropertyExample();   //shows how to set properties and read them
var
MyCar: TCar;                 // Will hold a TCar instance; used in the example subAuto
begin
MyCar := TCar.Create;        // Create the object
try
MyCar.Color:='Green';      // Sets the color property... which calls the setcolor function... which sets the object's FColor variable
showmessage(MyCar.Color);  // Now read the object property again... should show Green
finally
MyCar.Free;                // Free the memory for the object again, regardless of what errors occurred in the rest of the code
end;
end;

Objects as properties

You can assign objects as properties, e.g.:

type
TEngine = class
...
TCar = class
private
FMyEngine: TEngine;
public
implementation
// ...
// Code in TCar that creates and frees FMyEngine as needed
// (e.g. in the constructor and destructor)
// ...

Read access here means something that may not be obvious at first glance. Outside code has read only access to the pointer/reference to the TEngine object in question, which means it cannot create or destroy the object.

However, it can use this read access to access the properties of the FMyEngine object and change those.

If you wish to give calling code the responsibility of creating and destroying the object, you can also assign write access, e.g.

...
public
property Engine: TEngine read FMyEngine write FMyEngine;
...