Method pointers > 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).
 
// Wer noch Fragen hat / if you have questions: Florian.Benz@t-online.de
}

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