// 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
// incompatible 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 executable 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 function 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;