Mega Code Archive
How to Convert method pointers into function pointers
<![CDATA[
Title: How to Convert method pointers into function pointers
// 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).
]]>