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Pointers to a function in Delphi?

Posted on 2008-06-10
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Last Modified: 2010-05-18
I have an object, that sometimes needs to make a call to a function, and that function can be one of many. So i need to be able to tell the object what function it should use "this time". How do i:

(1) Make a var that holds the adress to a function.

(2) Tell the object what adress the function now have.

Im thinking like this:

// the function
function MyValue : integer
begin
result = 10;
end;

// the objec

procedure MyObject.SetFunction (p : funcpointer);


begin
       use p, how?
end;
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Question by:Sunsales
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7 Comments
 
LVL 28

Accepted Solution

by:
2266180 earned 500 total points
ID: 21753913

type
  TFunction=function:integer;
 
var f:TFunction;
 
function MyValue : integer;
begin
  result := 10;
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  f:=@myvalue;
  showmessage(inttostr(f));
end;

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Author Comment

by:Sunsales
ID: 21754325
Thanx, but:

(1) I have no type "tfunction". Unknown to Delphi.

(2) In form create i need to have a variable storing the function. FormCreate dont know what variant im using. It can be MyValue1 or Myvalue2 or a complete different function...
0
 
LVL 13

Expert Comment

by:ThievingSix
ID: 21754418
*Points to "type TFunction=function:integer;"*

You define the TFunction yourself.
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LVL 28

Expert Comment

by:2266180
ID: 21754425
(1) - it's only 15 lines of code. can't you pay more attention? I declared it for you

(2) - I don't understand your problem. declare your variable wherever you want.
0
 
LVL 13

Expert Comment

by:ThievingSix
ID: 21755019
Let's try this:
unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
 
type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
type
  TFunction = Function(Input: Integer): Integer;
 
var
  Form1 : TForm1;
  FunctionToCall : TFunction;
 
implementation
 
{$R *.dfm}
 
 
function ThisisFunctionA(Input: Integer): Integer;
begin
  Result := Input - 1;
end;
 
function ThisisFunctionB(Input: Integer): Integer;
begin
  Result := Input + 1;
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  FunctionToCall := @ThisisFunctionA;
end;
 
procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(IntToStr(FunctionToCall(1)));
  FunctionToCall := @ThisisFunctionB;
  ShowMessage(IntToStr(FunctionToCall(1)));
end;
 
end.

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LVL 37

Expert Comment

by:Geert Gruwez
ID: 21756659
guess you want to use methods instead of ordinary procedures ?

button 1 changes the value with 5

button 2 changes the proc called to function A

button 3 changes the proc called to function B
unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
 
type
  TFunction = Function(Input: Integer): Integer of object;
 
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
  private
    FunctionToCall: TFunction;
    fValue: integer;
    function ThisisFunctionA(Delta: Integer): Integer;
    function ThisisFunctionB(Delta: Integer): Integer;
  public
    property aValue: integer read fValue write fValue;
  end;
 
var
  Form1 : TForm1;
 
implementation
 
{$R *.dfm}
 
 
function TForm1.ThisisFunctionA(Delta: Integer): Integer;
begin
  Result := fValue - Delta;
end;
 
function TForm1.ThisisFunctionB(Delta: Integer): Integer;
begin
  Result := fValue + Delta;
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  fValue := 100;
  FunctionToCall := ThisisFunctionA;
end;
 
procedure TForm1.Button1Click(Sender: TObject);
begin
  FunctionToCall(5);
  ShowMessage(IntToStr(AValue));
end;
 
procedure TForm1.Button2Click(Sender: TObject);
begin
  FunctionToCall := ThisisFunctionA;
end;
 
procedure TForm1.Button3Click(Sender: TObject);
begin
  FunctionToCall := ThisisFunctionB;
end;
 
end.

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Author Comment

by:Sunsales
ID: 21760362
Thanx all, first example is god (all are) , i only sleep a few hours so i not c...
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