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Solved
need help with "MethodAddress"
Posted on 2000-03-08
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!
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pls show here your codes for us to understand your situation.
"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.
"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.
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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