This only makes sense if the passedin value is of type idispatch or iunknown. Whats the difference between thread and process, exe, dll and mtdll, earlybinding and latebinding, between vtable and idispatch. Now, my issue is, the com object uses the idispatch and iunknown interfaces of the object i pass to determine what events to send and to actually send the events. The idispatch interface exposes a number of methods for automation clients that require latebinding such as scripting languages. It provides the basic ability to cast a instance of a com object to any of its supported interfaces. However, dispatch is more common in american english and despatch seems to be. This session requires basic knowledge of memory adresses, pointers and data storage. But just as you no longer need to understand assembly language to get software working these days, pretty soon, youll no longer need to understand all the nuances of iunknown and com to. But if you want to do that you can use queryinterface to get idispatch pointer but that is not converting you will be getting a new pointer. Of course, you can provide access through both idispatch and through a custom interface, but that involves bookkeeping.
The difference between assignment and attachment with atl. Net framework view topic how to implement idispatch. Now instead of deriving from iunknown, the isum interface derives from idispatch, which is itself derived from iunknown, so we still get the queryinterface, addref, and release methods added to the vtable. How can i specify my own implementations of these two interfaces, so that they get called instead of the default system implementation. It is the foundation upon which ole and activex are based, and provides a means to reuse code without requiring recompilation. So i think it unlikely that the iideaclientopendatabase method returns anything other than idispatch.
Automation clients use the two methods getidsofnames and invoke to achieve late binding. When the property is an iunknown,idispatch, or safearray, the flag. Com component object model com is microsofts component software architecture developed primarily for windows. Use early binding and late binding in automation office. Implement the idispatch interface in comenabled classes that have to act as automation controllers. The iunknown interface lets clients get pointers to other interfaces on a given object through the queryinterface method, and manage the existence of the object through the iunknown addref and iunknown release methods. Note the only difference between com and dcom is the connection between the client and the server. To implement multiinterface dispatch on one or more pure iunknown derived interfaces, use puredispatchimpl.
It is a generally good idea to have an answer for this question if you claim you know com in your resume. Iunknown methods can be used to switch between interfaces on an object, add references, and release objects. This chapters deals with the similarities and differences between objects and interfaces. While its a very simple interface, compared to some of the others well get to later, it does highlight a number of interesting and important behaviors that the. What is the difference between idispatch and iunkown in com. The following is a list of tips and tricks that might be useful to com developers of all levels. For automation and active server objects, the wizard implements iunknown and idispatch, which provides automatic marshaling. Iunknown and retval rewriting today, were going to finish out the definition of iuknown that we started last time, by adding the three methods to the interface. However, the efficiency difference between using a unicode string and an. Vba can define a parameter as being of type iunknown by using a reference to the external library stdole2.
The separation between declaration and implementation is discussed. I can get their window handles via regular api calls, and can get other basic info about them based on the api calls as well, but need to actually obtain a pointer to the object so that i can manipulate it, etc. If we dont use object and use a comimport interface you. In addition to the methods inherited from iunknown, server developers must implement the following methods within the class definition of each object that is exposed. Idispatch is the interface that exposes the ole automation protocol. Difference between ccomptr and ccomqiptr 140 difference between ccomptr and ccomqiptr vcwebbrowserie mfc activex com. What is the difference between method and function. The documentation goes into detail here, but the gist is that object by default marshals as variant, which is almost always not what we want. This is the only real difference between a dispinterface and a dual interface. Calling functions and accessing properties apache openoffice wiki. But if you want to do that you can use queryinterface to get idispatch pointer but that. According to the oxford dictionary, both spellings are synonymous for all meanings of the word. There is a special type of com server, where the com objects support the idispatch interface, called an automation server. And when we declare dispinterface someinterce, then it means.
Notice that in the idl fragment, the syntax seems more standard. Well the best way is to get your idispatch interface directly with its clsid. In addition, atl appears to be getting easier to use as it matures. In this video we learn about the iunknown interface the one interface all com objects must implement. It is important to note that what is late bound is the function being called and not the way it is called. Idispatch, the main interface in automation is described. All other interfaces must derive directly or indirectly from iunknown. When the ccomptr is destructed, it will release the pointer, undoing the addref that was performed by the assignment operator.
Tinterfacedobject implements the three iunknown methods already so that is a suitable class to inherit from, however we must reimplement the queryinterface method to indicate runtime support for the events interface as well. It is through the iunknown interface that clients maintain a reference on an. Cdispatchptr is a subclass of idispatchptr, with the getsetsetrefinvoke methods added. Hi, whats the difference between an interface declared with. Gettypeinfocount can be 1 or 0 and represents the number of type information interfaces that the object provides. Class objects in an exe server has one difference from those in a dll server. Idispatch interface and accessibility win32 apps microsoft docs. Createcomobject returns an object of the iunknown type, whereas createoleobject returns an object of the idispatch type. What methods are provided by iunknown answer cheruku raghu. The object data types is the core connection between vba and com. All object manipulation through strict interfaces polymorphism iunknown via multiple interfaces per class idispatch discoverable. Otherwise, you may consider your interview failed at this point. Only some classes support idispatch which derives from iunknown. If you have an interesting tip thats not listed here, please submit it to me and ill add it to the list if i think its useful.
Can somebody tell me what is the difference between the methods and the functions in atl. Idispatch derives from iunknown and extends its set of three methods addref, release and queryinterface with an additional four methods gettypeinfocount, gettypeinfo, getidsofnames and invoke. From msdn, dispinterface is another interface inherits from idispatch. To call a method in a simple iunknown interface all you need is to know where in the vtable the function pointer is. Idispatch builds on iunknown adding the facility to discover at runtime the attributes and interfaces supported by the specified com object. Idispatchimplex template classes for multiinterface. Iunknown is the base interface for every com class. This type of implementation creates what is known as a singleton object since there is.
Lets start with a quick recapitulation of object oriented programming. The idispatch interface allows many scripting languages to control the com objects methods and properties using runtime method calls to idispatch a method call dispatching interface. The differences between the two architectures are fairly stark. Visual foxpro developers make no differences between defining a class and implementing it. An idispatch interface is a special type of iunknown interface that includes methods that allow methods to be called by name at runtime. The two that you need to know about are idispatch interfaces and idl. Format differences like byte ordering and number representations are bridged this way.
The idispatch interface has methods that allow clients to ask the object what. Programmers are not allowed to use an iunknown data type from vba this is a hidden data type. Iunknown implementations are generally the same for every com class you encounter the main difference between them is the interfaces exposed by each class. The essence of automation objects is the idispatch interface. Every com class implements an interface named iunknown. What is the difference between dispatch and despatch answers. It provides the basic ability to cast a instance of a com object to any of its supported interfaces queryinterface. Implementing the idispatch interface microsoft docs. Dispinterface is an idispatch plus an agreement on what each dispid means and what parameters it should come with.
The idispatch interface was initially designed to support automation. Observe that assigning a t to a ccomptr addrefs the incoming pointer and releases the old pointer if any. Generally, mfc enables you to get your project up and running more quickly, but sacrifices flexibility. As in the case of iunknown, visual basic programmers never deal with this interface directly. There is a requirement, however, that there is type info available for the interfaces.
Atl isnt quite as quick and easy to use, but it is comfriendly. There is another difference between these two creation functions. The idispatch interface extends the iunknown interface and is a sort of gateway interface to many more interfaces grimes97. Extending iunknown, it is one of the standard interfaces that can be exposed by com objects. Explain some of commonly used methods provided by iunknown. The implementation adds a single idispatch implementation to the class, and uses typeinfo to dispatch against the pure interfaces. In atl, idispatchinvoke is implemented by idispatchimpl. The interface header also includes the dual attribute to indicate a dual interface.
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