_Struct is a class that is used to get easy access to structures and its members. sizeof() is used to calculate structure size and is mandatory for _Struct to work. Read about usage of _Struct here: http://www.autohotkey.net/~HotKeyIt/AutoHotkey/_Struct.htm Read about usage of sizeof here: http://www.autohotkey.net/~HotKeyIt/AutoHotkey/sizeof.htm Download _Struct.ahk and sizeof.ahk to your library (Path of AutoHotkey.exe\Lib\_Struct.ahk...) Visit _Struct on AutoHotkey forum http://www.autohotkey.com/forum/viewtopic.php?p=364838#364838! Companion projects: [WinStructs](https://github.com/ahkscript/WinStructs) - A collection of definitions for Windows Structures. If you create structure definitions, please add them here so only one person ever need do a structure. [AHK-SizeOf-Checker](https://github.com/ahkscript/AHK-SizeOf-Checker) - Verifies sizes of structures that _Struct creates against the size of the structure reported in compiled C++ code. REQUIRES VISUAL STUDIO.