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need help with "MethodAddress"
hi all:
i wanna get a function's address by MethodAddress, and then call it.
But, in the function, if i access a VCL(eg. form,memo), i always get a "Access violation" error. if dont access VCL, it works fine.
how to resolve it? thanks!
i wanna get a function's address by MethodAddress, and then call it.
But, in the function, if i access a VCL(eg. form,memo), i always get a "Access violation" error. if dont access VCL, it works fine.
how to resolve it? thanks!
Asked:
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5
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4
1 Solution
OK...
Methodaddress returns a pointer to the code for that published method in a class. But to call it you must be in the context of an instance of the class.
You can use a TMethod to hold the pointer to the instance data and the pointer to a methodaddress result.
Here is an example:
unit Unit18;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
published
procedure a;
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.a;
begin
ShowMessage('!');
end;
procedure TForm1.Button1Click(Sender
type
t = procedure of object;
var
x : tmethod;
y : t;
begin
x.data := @self;
x.code := self.methodaddress('a');
y := t(x);
y;
end;
end.
Cheers,
Raymond.
Methodaddress returns a pointer to the code for that published method in a class. But to call it you must be in the context of an instance of the class.
You can use a TMethod to hold the pointer to the instance data and the pointer to a methodaddress result.
Here is an example:
unit Unit18;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
published
procedure a;
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.a;
begin
ShowMessage('!');
end;
procedure TForm1.Button1Click(Sender
type
t = procedure of object;
var
x : tmethod;
y : t;
begin
x.data := @self;
x.code := self.methodaddress('a');
y := t(x);
y;
end;
end.
Cheers,
Raymond.
Where I Said: "But to call it you must be in the context of an instance of the class." I really meant you need to have the context of an instance, you don't need to be in the instance itself to call it.
Cheers,
Raymond.
Cheers,
Raymond.
i do as this:
type
tform1=class(tform)
....
published
procedure abc;
end;
....
procedure tform1.abc;
begin
//error occur at this sentence
memo1.lines.add('abc');
end;
procedure tform1.buttonclick(...);
var func:procedure;
begin
func:=MethodAddress('abc')
func;
end;
type
tform1=class(tform)
....
published
procedure abc;
end;
....
procedure tform1.abc;
begin
//error occur at this sentence
memo1.lines.add('abc');
end;
procedure tform1.buttonclick(...);
var func:procedure;
begin
func:=MethodAddress('abc')
func;
end;
rwilson :
in your sample code, drag a TMemo to your form, and replace "showmessage" with "memo1.lines.add(...);"
the error will occur.
i means: if you access a VCL in the function, it dont work.
in your sample code, drag a TMemo to your form, and replace "showmessage" with "memo1.lines.add(...);"
the error will occur.
i means: if you access a VCL in the function, it dont work.
Whoops: I didn't need to take the address of self, just cast it to a pointer. The following works:
unit Unit6;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
published
procedure a;
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.a;
begin
memo1.lines.add('...');
end;
procedure TForm1.Button1Click(Sender
type
t = procedure of object;
var
x : tmethod;
y : t;
begin
x.data := pointer(self);
x.code := self.methodaddress('a');
y := t(x);
y;
end;
Cheers,
Raymond.
unit Unit6;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
published
procedure a;
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.a;
begin
memo1.lines.add('...');
end;
procedure TForm1.Button1Click(Sender
type
t = procedure of object;
var
x : tmethod;
y : t;
begin
x.data := pointer(self);
x.code := self.methodaddress('a');
y := t(x);
y;
end;
Cheers,
Raymond.
rwilson: your code works. but, if i want add parameters to function "a", how to do that?
eg: procedure a(s:string)
eg: procedure a(s:string)
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"Access voilation" means you are accessing address was not allocated to your program.
it often happened when you code that in a function:
in method
procedure TForm1.metho1();
begin
//Form1.property1:=somethi
//Form1.method2();
//something like that!!!
end;
...
var F:TForm1;
F:=TForm1.Create(applicati
F.method1;
it means Form1 is not created but want to call it.