_OFFSET - QB64Official/qb64 GitHub Wiki

The _OFFSET variable type stores the location of a value in memory. The byte size varies as required by the system.

Syntax

DIM variable AS _OFFSET

Description

  • _OFFSET types can be created as signed or _UNSIGNED at the programmer's discretion.
  • The type suffix for _OFFSET is %& which designates the integer value's flexible size.
  • Offset values are only useful when used in conjunction with _MEM or DECLARE LIBRARY procedures.
  • OFFSET values are used as a part of the _MEM variable Variable Types in QB64. Variable.OFFSET returns or sets the current position in memory.
  • API DECLARE LIBRARY parameter or TYPE names may include lp, ptr or p which designates them as a pointer type.
  • Warning: _OFFSET values cannot be cast to other variable type values reliably.
  • Warning: Variable length STRING values can move about in memory at any time. If you get the _OFFSET of a variable length sting on one code line and use it on the next it may not be there anymore.** To be safe, move variable length strings into fixed length strings first.**

Example(s)

The SHBrowseForFolder function receives information about the folder selected by the user in Windows XP and 7.


DECLARE CUSTOMTYPE LIBRARY
    FUNCTION FindWindow& (BYVAL ClassName AS _OFFSET, WindowName$)
END DECLARE

_TITLE "Super Window"
hwnd& = FindWindow(0, "Super Window" + CHR$(0))

TYPE BROWSEINFO  'typedef struct _browseinfo '[Microsoft MSDN](http://msdn.microsoft.com/en-us/library/bb773205%28v=vs.85%29.aspx)
  hwndOwner AS LONG '              '  HWND 
  pidlRoot AS _OFFSET '            '  PCIDLIST_ABSOLUTE
  pszDisplayName AS _OFFSET '      '  LPTSTR 
  lpszTitle AS _OFFSET '           '  LPCTSTR       
  ulFlags AS _UNSIGNED LONG        '  UINT   
  lpfn AS _OFFSET '                '  BFFCALLBACK  
  lParam AS _OFFSET '              '  LPARAM  
  iImage AS LONG '                 '  int  
END TYPE  'BROWSEINFO, *PBROWSEINFO, *LPBROWSEINFO;

DECLARE DYNAMIC LIBRARY "shell32"
  FUNCTION SHBrowseForFolder%& (x AS BROWSEINFO) '[Microsoft MSDN](http://msdn.microsoft.com/en-us/library/bb762115%28v=vs.85%29.aspx)
  SUB SHGetPathFromIDList (BYVAL lpItem AS _OFFSET, BYVAL szDir AS _OFFSET) '[Microsoft MSDN](http://msdn.microsoft.com/en-us/library/bb762194%28VS.85%29.aspx)
END DECLARE

DIM b AS BROWSEINFO
b.hwndOwner = hwnd
DIM s AS STRING * 1024
b.pszDisplayName = _OFFSET(s$)
a$ = "Choose a folder!!!" + CHR$(0)
b.lpszTitle = _OFFSET(a$)
DIM o AS _OFFSET
o = SHBrowseForFolder(b)
IF o THEN
    PRINT LEFT$(s$, INSTR(s$, CHR$(0)) - 1)
    DIM s2 AS STRING * 1024
    SHGetPathFromIDList o, _OFFSET(s2$)
    PRINT LEFT$(s2$, INSTR(s2$, CHR$(0)) - 1)
ELSE
    PRINT "Cancel?"
END IF

See Also