JCL中由接口获得对象的方法

我原本并不想单独发以下JCL的方法，可是想到很多人可能还是不知道的。另外，从我博客的回复上看，很多人也愿意去了解一下，特此贴出JCL中的代码。

//=== Interface information ==================================================



function GetImplementorOfInterface(const I: IInterface): TObject;




...{ TODO -cDOC : Original code by Hallvard Vassbotn }




...{ TODO -cTesting : Check the implemetation for any further version of compiler }


const


  AddByte = $04244483; // opcode for ADD DWORD PTR [ESP+4], Shortint


  AddLong = $04244481; // opcode for ADD DWORD PTR [ESP+4], Longint


type


  PAdjustSelfThunk = ^TAdjustSelfThunk;


  TAdjustSelfThunk = packed record


    case AddInstruction: Longint of


      AddByte: (AdjustmentByte: ShortInt);


      AddLong: (AdjustmentLong: Longint);


  end;


  PInterfaceMT = ^TInterfaceMT;


  TInterfaceMT = packed record


    QueryInterfaceThunk: PAdjustSelfThunk;


  end;


  TInterfaceRef = ^PInterfaceMT;


var


  QueryInterfaceThunk: PAdjustSelfThunk;


begin


  try


    Result := Pointer(I);


    if Assigned(Result) then


    begin


      QueryInterfaceThunk := TInterfaceRef(I)^.QueryInterfaceThunk;


      case QueryInterfaceThunk.AddInstruction of


        AddByte:


          Inc(PChar(Result), QueryInterfaceThunk.AdjustmentByte);


        AddLong:


          Inc(PChar(Result), QueryInterfaceThunk.AdjustmentLong);


      else


        Result := nil;


      end;


    end;


  except


    Result := nil;


  end;


end;

注释：这段代码出自JCL代码中JclSysUtils单元中。
提示：读这段代码，要明白一个道理，在代码执行过程中，接口和对象指针的偏移是硬编码的在汇编中的。转换的过程，就是解析这段汇编的过程。