JCL中由接口获得对象的方法

2007年05月15日 21:43:00
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。

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