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Как преобразовать указатель на метод в указатель на функцию?

01.01.2007
// 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; 

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