Solved

Pointers to a function in Delphi?

Posted on 2008-06-10
7
185 Views
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;
0
Comment
Question by:Sunsales
  • 2
  • 2
  • 2
  • +1
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;

Open in new window

0
 

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.
0
ScreenConnect 6.0 Free Trial

Check out the updates in one game-changing release, ScreenConnect 6.0, based on partner feedback. New features include a redesigned UI that improves session organization and overall user experience. See the enhancements for yourself!

 
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.

Open in new window

0
 
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.

Open in new window

0
 

Author Comment

by:Sunsales
ID: 21760362
Thanx all, first example is god (all are) , i only sleep a few hours so i not c...
0

Featured Post

Optimizing Cloud Backup for Low Bandwidth

With cloud storage prices going down a growing number of SMBs start to use it for backup storage. Unfortunately, business data volume rarely fits the average Internet speed. This article provides an overview of main Internet speed challenges and reveals backup best practices.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Have you ever had your Delphi form/application just hanging while waiting for data to load? This is the article to read if you want to learn some things about adding threads for data loading in the background. First, I'll setup a general applica…
Hello everybody This Article will show you how to validate number with TEdit control, What's the TEdit control? TEdit is a standard Windows edit control on a form, it allows to user to write, read and copy/paste single line of text. Usua…
This Micro Tutorial will give you a basic overview how to record your screen with Microsoft Expression Encoder. This program is still free and open for the public to download. This will be demonstrated using Microsoft Expression Encoder 4.
Along with being a a promotional video for my three-day Annielytics Dashboard Seminor, this Micro Tutorial is an intro to Google Analytics API data.

773 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question