Code:

{

Converting method pointers into function pointers

Often you need a function pointer for a callback function. But what,

if you want to specify a method as an callback? Converting a method

pointer to a function pointer is not a trivial task; both types are

incomatible with each other. Although you have the possibility to

convert like this "@TClass.SomeMethod", this is more a hack than a

solution, because it restricts the use of this method to some kind

of a class function, where you cannot access instance variables.

If you fail to do so, you'll get a wonderful gpf. But there is

a better solution: run time code generation! Just allocate an

executeable memory block, and write 4 machine code instructions into

it: 2 instructions loads the two pointers of the method pointer

(code & data) into the registers, one calls the method via the code

pointer, and the last is just a return Now you can use this pointer

to the allocated memory as a plain funtion pointer, but in fact you are

calling a method for a specific instance of a Class.

}

type TMyMethod = procedure of object;

function MakeProcInstance(M: TMethod): Pointer;

begin

// allocate memory

GetMem(Result, 15);

asm

   // MOV ECX,

   MOV BYTE PTR [EAX], $B9

   MOV ECX, M.Data

   MOV DWORD PTR [EAX+$1], ECX

   // POP EDX

   MOV BYTE PTR [EAX+$5], $5A

   // PUSH ECX

   MOV BYTE PTR [EAX+$6], $51

   // PUSH EDX

   MOV BYTE PTR [EAX+$7], $52

   // MOV ECX,

   MOV BYTE PTR [EAX+$8], $B9

   MOV ECX, M.Code

   MOV DWORD PTR [EAX+$9], ECX

   // JMP ECX

   MOV BYTE PTR [EAX+$D], $FF

   MOV BYTE PTR [EAX+$E], $E1

end;

end;

procedure FreeProcInstance(ProcInstance: Pointer);

begin

// free memory

FreeMem(ProcInstance, 15);

end;

{

After all, you should not forget to release the allocated memory.

"TMyMethod" can be modified according your specific needs, e.g.

add some parameters for a WindowProc. N.B.: Yes, I know, Delphi

has those "MakeProcInstance" function in its forms unit. But this

works a little bit different, has much more overhead, and most

important, you have to use the forms unit, which increases the

size of your exe drastically, if all other code doesn't use the

VCL (e.g. in a fullscreen DirectX/OpenGl app).

// Wer noch Fragen hat / if you have questions: Florian.Benz@t-online.de

}