Method pointers в function pointers
Материал из DRKB
Method pointers --> function pointers[править | править код]
{
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
}
Language: en
Source: Взято с сайта http://www.swissdelphicenter.ch/en/tipsindex.php
ID: 00259