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73 lines
3.1 KiB
Markdown
73 lines
3.1 KiB
Markdown
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The [_OFFSET](_OFFSET) variable type stores the location of a value in memory. The byte size varies as required by the system.
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## Syntax
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> [DIM](DIM) variable [AS](AS) **[_OFFSET](_OFFSET)**
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## Description
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* [_OFFSET](_OFFSET) types can be created as signed or [_UNSIGNED](_UNSIGNED) at the programmer's discretion.
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* The type suffix for [_OFFSET](_OFFSET) is **%&** which designates the integer value's flexible size.
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* Offset values are only useful when used in conjunction with [_MEM](_MEM) or [DECLARE LIBRARY](DECLARE-LIBRARY) procedures.
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* OFFSET values are used as a part of the [_MEM](_MEM) variable [Variable Types](Variable-Types) in QB64. Variable.OFFSET returns or sets the current position in memory.
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* API [DECLARE LIBRARY](DECLARE-LIBRARY) parameter or [TYPE](TYPE) names may include **lp, ptr** or **p** which designates them as a pointer type.
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* **Warning: [_OFFSET](_OFFSET) values cannot be cast to other variable type values reliably.**
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* **Warning: Variable length [STRING](STRING) values can move about in memory at any time.** If you get the [_OFFSET](_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.**
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## Example(s)
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The SHBrowseForFolder function receives information about the folder selected by the user in Windows XP and 7.
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```vb
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DECLARE CUSTOMTYPE LIBRARY
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FUNCTION FindWindow& (BYVAL ClassName AS _OFFSET, WindowName$)
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END DECLARE
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_TITLE "Super Window"
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hwnd& = FindWindow(0, "Super Window" + CHR$(0))
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TYPE BROWSEINFO 'typedef struct _browseinfo '[Microsoft MSDN](http://msdn.microsoft.com/en-us/library/bb773205%28v=vs.85%29.aspx)
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hwndOwner AS LONG ' ' HWND
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pidlRoot AS _OFFSET ' ' PCIDLIST_ABSOLUTE
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pszDisplayName AS _OFFSET ' ' LPTSTR
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lpszTitle AS _OFFSET ' ' LPCTSTR
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ulFlags AS _UNSIGNED LONG ' UINT
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lpfn AS _OFFSET ' ' BFFCALLBACK
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lParam AS _OFFSET ' ' LPARAM
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iImage AS LONG ' ' int
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END TYPE 'BROWSEINFO, *PBROWSEINFO, *LPBROWSEINFO;
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DECLARE DYNAMIC LIBRARY "shell32"
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FUNCTION SHBrowseForFolder%& (x AS BROWSEINFO) '[Microsoft MSDN](http://msdn.microsoft.com/en-us/library/bb762115%28v=vs.85%29.aspx)
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SUB SHGetPathFromIDList (BYVAL lpItem AS _OFFSET, BYVAL szDir AS _OFFSET) '[Microsoft MSDN](http://msdn.microsoft.com/en-us/library/bb762194%28VS.85%29.aspx)
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END DECLARE
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DIM b AS BROWSEINFO
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b.hwndOwner = hwnd
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DIM s AS STRING * 1024
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b.pszDisplayName = _OFFSET(s$)
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a$ = "Choose a folder!!!" + CHR$(0)
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b.lpszTitle = _OFFSET(a$)
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DIM o AS _OFFSET
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o = SHBrowseForFolder(b)
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IF o THEN
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PRINT LEFT$(s$, INSTR(s$, CHR$(0)) - 1)
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DIM s2 AS STRING * 1024
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SHGetPathFromIDList o, _OFFSET(s2$)
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PRINT LEFT$(s2$, INSTR(s2$, CHR$(0)) - 1)
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ELSE
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PRINT "Cancel?"
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END IF
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```
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## See Also
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* [_WINDOWHANDLE](_WINDOWHANDLE)
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* [Using _OFFSET](Using--OFFSET)
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* [_OFFSET (function)](_OFFSET-(function)), [_MEM](_MEM)
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* [DECLARE LIBRARY](DECLARE-LIBRARY)
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* [DECLARE DYNAMIC LIBRARY](DECLARE-DYNAMIC-LIBRARY)
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* [Variable Types](Variable-Types)
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