Why does this code declare a DISTINCT type?

StackOverflow https://stackoverflow.com/questions/8955349

  •  18-04-2021
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Question

ShlObj.pas line 9922 (as in Delphi XE):

type
  BFFCALLBACK = function(Wnd: HWND; uMsg: UINT; lParam, lpData: LPARAM): Integer stdcall;
  TFNBFFCallBack = type BFFCALLBACK;
  {$EXTERNALSYM BFFCALLBACK}

Answering to David Heffernan's comment here because i think this outlines a relevant background for possible why do you ask this? question. Procedural types for callback routines of Windows API origin are declared as type aliases (you know, canonical API name + "Pascalised" alias, or vice versa sometimes). Examples are Windows.PIMAGE_TLS_CALLBACK, ShlObj.LPFNDFMCALLBACK et cetera. Type declaration shown above is an exception. Thats why i'm asking :-)

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Solution

It is just a way to force us programmers to use TFNBFFCallBack instead of BFFCALLBACK. If it was an alias we could use either for the browse info structure. On a different platform TFNBFFCallBack could point to something different than BFFCALLBACK.

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