Is using System.Assigned () a trap? What is your opinion?

 


unit UnitMyClassX;


interface


uses


Winapi.Windows,


Winapi.Messages,


System.SysUtils,


System.Variants,


System.Classes,


Vcl.Graphics,


Vcl.Controls,


Vcl.Forms,


Vcl.Dialogs,


Vcl.StdCtrls;


type


TMyClassX = class


private


FMyName: String;


procedure SetMyName(const Value: String);


public


property MyName: String read FMyName write SetMyName;


end;


//


TForm1 = class(TForm)


Button1: TButton;


Memo1: TMemo;


procedure Button1Click(Sender: TObject);


private


procedure prcMyLog(lText:String);


public


{ Public declarations }


end;


var


Form1: TForm1;


implementation


{$R *.dfm}


procedure TForm1.prcMyLog(lText:String);


begin


Memo1.Lines.Add(lText);


Memo1.Lines.Add('');


end;


type


TMyFunction = function: Pointer; // better to scope!


// var


// LMyFunction: TMyFunction = MyFunction; // dont works here! MyFunction is below!


function MyFunction: Pointer; // its ok! now!


begin


Result := nil;


end;


// type


// TMyFunction = function: Pointer; // works too!


var


LMyFunction: TMyFunction = MyFunction; // here is accepted! works likes Global (to down) use for this implementation!


{


function MyFunction: Pointer; // its not OK, because my "var" definition above!


begin


Result := nil;


end;


}


procedure TMyClassX.SetMyName(const Value: String);


begin


FMyName := Value;


end;


procedure TForm1.Button1Click(Sender: TObject);


var


lMyClassX: TMyClassX;


//


LNotifyEvent: TNotifyEvent;


//


// LMyFunction: TMyFunction = MyFunction; // cannot initialize local variables!


//


begin


{ On HELP SYSTEM:


Tests for a "nil" (unassigned) pointer or procedural variable.


Use "Assigned" to determine whether the pointer or the procedure referenced by "P is nil".


"P" must be a "variable reference of a pointer or procedural type".


"Assigned(P)" corresponds to the test "P <> nil" for a "pointer variable",


and "@P <> nil" for a "procedural variable".


"Assigned" returns False if "P is nil", True otherwise.


Tip: When testing object events and procedures for assignment,


you cannot test for "nil", and using "Assigned" is the right way.


}


//


// note that my variables is not explicitly assigned!


//


Memo1.Clear;


//


if Assigned(lMyClassX) then


prcMyLog('lMyClassX is Assigned') { <-- is returning this line, is TRUE }


else


prcMyLog('lMyClassX is Not Assigned');


//


if (lMyClassX <> nil) then


prcMyLog('lMyClassX is Assigned') { <-- is returning this line, is TRUE }


else


prcMyLog('lMyClassX is Not Assigned');


//


// testing my NotifyEvent variable:


//


if Assigned(LNotifyEvent) then


prcMyLog('lMyNotifyEvent is Assigned') { <-- is returning this line, is TRUE }


else


prcMyLog('lMyNotifyEvent is Not Assigned');


//


// --> "LNotifyEvent" dont appears on "Ctrl+Space" for choice (likes "lMyClassX")


// --> but same forcing this expression i have this error:


//


// if (LNotifyEvent <> nil) then --> E2035 Not enought actual parameters ...


// --> F2063 Could not compile used unit "UnitMyClassX.pas"


{ On Help System:


Tip: In particular cases there is a difference between using "Assigned(Something)"


and "Something <> nil".


}


//


// ******************************************************************************


{


For instance, in the following code "Assigned(LMyFunction)" checks whether


the "LMyFunction variable" is "actually assigned",


while "LMyFunction<> nil" tests "the result value of LMyFunction for assignment".


}


if (LMyFunction <> nil) then


prcMyLog('Statement "LMyFunction <> nil"')


else


prcMyLog('Statement "LMyFunction = nil"'); { <-- is returning this line, is FALSE }


//


if Assigned(LMyFunction) then


prcMyLog('Statement "Assigned(LMyFunction)" = true') { <-- is returning this line, is TRUE }


else


prcMyLog('Statement "Assigned(LMyFunction)" = false');


//


{ on Help System:


Note: "Assigned" cannot detect a dangling pointer --that is, "one that is not nil",


but that "no longer points to valid data".


For example, in the code example for "Assigned(SystemAssigned)",


Assigned does "not detect that P is not valid".


}


//


// testing my "Form1" variable"


//


if (Form1 <> nil) then


prcMyLog('Statement "Form1 <> nil"') { <-- is returning this line, is TRUE }


else


prcMyLog('Statement "Form1 = nil"');


//


if Assigned(Form1) then


prcMyLog('Statement "Assigned(Form1)" = true') { <-- is returning this line, is TRUE }


else


prcMyLog('Statement "Assigned(Form1)" = false');


//


//


if (TForm1 <> nil) then


prcMyLog('Statement "TForm1 <> nil"') { <-- is returning this line, is TRUE }


else


prcMyLog('Statement "TForm1 = nil"');


//


(*


if Assigned(TForm1) then // E2036 Variable required --> on compile!


prcMyLog('Statement "Assigned(TForm1)" = true')


else


prcMyLog('Statement "Assigned(TForm1)" = false');


*)


//


end;


end.