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project's internal child references also have a valid parent reference, you don't even have to call SetParentPtr 0 on each child when the parent object is destroyed You know the parent can't be destroyed when an external child reference is still outstanding, so the very fact that the parent is being destroyed means that none of the child object's public functions will be called again This makes it unnecessary to clean up the weak references Clearly, this requires discipline in your internal code: You have to make sure that the parent holds the only remaining child references But such discipline can significantly simplify your teardown code and provide a performance boost as well With the code above, the only external evidence that you're using a wrapper object is that the child object's ObjPtr may change over time Also note that the aggregator uses blind vtable-delegation internally There is a very small performance impact, but the overhead is negligible, particularly when compared to the Implements and forwarding code you would have to write to achieve the same effect
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Publicly exposed objects are plentiful in the COM landscape One of your jobs as a VB programmer is to add to the set of public objects that you and other members of your organization can leverage to build applications Understanding how to create external objects is an important step in using them This chapter looks at how VB creates external objects using New, CreateObject, and GetObject; and API-based alternatives to object creation
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Before it can create an external COM object, VB must first be able to locate the object in the system The HKEY_CLASSES_ROOT\CLSID registry section maps a CLSID (class identifier) to a physical location on the local system or a remote system A CLSID is just one way COM uses a GUID (globally unique identifier), a 128-bit structure guaranteed to be unique across all systems In order to support external object creation, an ActiveX server registers a CLSID for each supported object and publishes a list of the supported classes in the server's typelib The typelib, which is generally built into the server's executable file as a resource, is a description of the supported classes in a library and the interfaces supported by those classes VB uses two mechanisms for providing a CLSID to COM, allowing COM to create an external object In the first, VB adds a project reference to the typelib that contains the CLSID This is the preferred mechanism for object creation; the
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EXTERNAL OBJECT CREATION
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compiled VB code knows which CLSID it must create and does not need to search for it In the second mechanism, VB looks up the CLSID at runtime based on a friendly name for the object (called a ProgID) A ProgID generally takes the form of ServerNameClassName This string is also stored in the system registry and simply points to the CLSID key Unfortunately, the ProgID string value is not entirely trustworthy because there is no guarantee that the string is unique If the consumers of a particular ActiveX server rely on the ProgID, the external objects are effectively removed from the system if another ActiveX server happens to register the same ProgID The key's potential duplication and the extra registry lookup required makes it best to use a CLSID directly whenever possible If you use VB's New keyword, the compiled code always knows the CLSID at runtime If you use CreateObject, VB finds the CLSID by using the ProgID stored in the registry This generates an extra system call for each object you create Using CreateObject also makes you vulnerable to naming conflicts because a ProgID is not a unique identifier New and CreateObject also return different initial interfaces In the case of New, the first interface requested is the default interface of the class, as described in the typelib If you create an object on another machine, process, or thread, you want to request only the interfaces you need because requesting additional interfaces is expensive However, CreateObject returns the IDispatch (As Object) interface or the (Unknown interface if IDispatch is not available Any request for an additional interface including the object's primary interface must be made with a subsequent Set statement In most uses, New and CreateObject are adequate for external COM-object creation With VB6's introduction of a second machine-name parameter to CreateObject, you can specify the DCOM server you would like the objects to be created on Although this new feature eliminates a common need for API work to create an object, the API still offers a great deal more flexibility than VB's built-in creation functions I'll look at how you can use VB's GetObject function and the CoGetClassObject API to locate a class factory for any CLSID This allows you to eliminate the bulk of the object-creation machinations from all but the first instance in which you create a specific class of object All the API and interface definitions you will need for this chapter's code are declared in
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