Persistence of Data
Why is Structured Storage a part of OLE, page 10
Structured Storage accurately describes what this compound document technology does, but why is this part of OLE? - the "Object Linking and Embedding" standard from Microsoft?
Part of this is the fact that OLE is in fact a collection of technologies - a sort of grab back under which many of the new developments for Windows falls. But part of it has to do with one of the major goals of OLE - the ability for users to deal with documents instead of applications.
Consider what happens when you embed an Excel spreadsheet into a Word document. When you click on the spreadsheet part of the document, the Excel user interface appears within Word - a technology called "in-place editing". The document is a word document, yet clearly, Word is able to somehow store and manipulate the Excel spreadsheet object that is somehow embedded into the Word file. Or can it?
In fact, Word has no idea what data is contained in the Excel spreadsheet or how to work with it. What actually happens is that the Word file is a compound document. One of the storages in the document is a place where objects foreign to Word are stored including, in this case, the Excel spreadsheet. Each object is stored in its own stream. When you save the word document, Word does not know how to store the spreadsheet object, but it does not need to - all it has to do is pass a handle to the stream to Excel. Excel then saves the spreadsheet data into the stream.
When you reload the document, Word looks at the objects in its object pool. Each of the streams starts with a GUID - a universal identifier. Word looks at that identifier and calls the OLE libraries asking it to create an object based on that stream information. OLE looks through the system registry, sees that it is an Excel spreadsheet object, then loads those parts of Excel that are necessary to display the object and allow the user to edit it.
As you can see, OLE Structured storage fills the critical need of allowing an application to store objects even though it does not know their internal format - it can simply pass a stream handle to a server that knows how to work with that object. You can use this same technique in Visual Basic in conjunction with the OLE container control to store foreign objects inside your own compound document files.
A quick look with a compound document browser shows that OLE Structured Storage is used by many Microsoft Applications including most of the Microsoft Office applications.
