Sub DisplayBookList()
    ObjectStore.BeginTrans
        Dim BookElt As IBookElt
        Dim Book as IBook
        Dim Author, Title As String
        Set BookElt = osDatabase.Value("Books", CBookElt)
        Do
            Set Book = BookElt.Book
            Author = Book.Author.Name
            Title = Book.Name
            List1.AddItem Author & " - " & Title
            Set BookElt = BookElt.Next
        Loop While Not BookElt Is Nothing
    ObjectStore.CommitTrans
End Sub

The first statement in the subroutine:

ObjectStore.BeginTrans

uses ObjectStore to start a transaction in the database. The last statement:

ObjectStore.CommitTrans

uses it to commit the transaction, freeing resources and allowing other processes to modify objects that were read from the database.

The following Set statement

Set BookElt = osDatabase.Value("Books", CBookElt)

passes CBookElt as an argument that specifies the class of BookElt, which is the named object or root object to be retrieved from the database. The CBookElt argument is used for type-checking purposes. Once the root object has been retrieved and type-checked, all further access to ObjectStore objects is automatically type-checked according to the declared Visual Basic types.

The Books Database

The Books database contains three types of objects, each of which is defined in books.ost as a C++ class: Author, Book, and BookElt. The following table lists both the C++ class definitions for each of these classes and the corresponding Visual Basic properties and syntax used to access them:


Visual Basic Code	Code in books.ost	C++ Class Definition
Dim Author As IAuthor Dim AuthorName as String Set Author = osDB("SampleAuthor").Value AuthorName = Author.Name	class Author { [propget] char* Name()data name; ... };	class Author { public: char *name; };
Dim Book As IBook Dim BookName as String Dim BookAuthor As IAuthor Set Book = osDB("SampleBook").Value BookName = Book.Name Set BookAuthor = Book.Author	class Book { [propget] char* Name()data name; [propget] Author* Author()data author; ... };	class Book { public: char* name; Author* author; };
Dim Books As IBookElt Dim Book As IBook Set Books = osDB("SampleBooks").Value Do Set Book = Books.Book Set Books = Books.Next Loop While Not Books Is Nothing	class BookElt { [propget] Book* Book()data book; [propget] BookElt* Next()data next; ... };	class BookElt { public: Book book; BookElt* next = NULL; };

The expression osDB("SampleAuthor").Value looks up the database root named SampleAuthor in the specified database and returns its value as an object that can be observed using its defined property names.

Note: Constants for the loop differ in C++ and Visual Basic. The C++ NULL pointer value is as translated Null in OLE and as Nothing in Visual Basic.

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Updated: 03/18/98 13:53:18

Chapter 1

Overview of the ActiveX Interface for ObjectStore

Windows 95 Patch Requirements

Global properties

A subroutine from the Visual Basic application