An interface is basically a class, containing only pure virtual functions and has no data members. For debugging purposes, its very useful to know where ano object begins in memory. A com object can be exposed to remote clients via the dcom protocol, in which case it. Interfaceisdual com interface types supports both idispatch and iunknown mode.
Another important method of iunknown interface is queryinterface. This interface defines a generic mechanism for setting properties on a codec encoder or decoder. Mar 07, 2014 this way we have achieved initial goal of iunknown interface. Iunknown is the interface containing queryinterface and also the reference counting methods addref and release which are used for com object lifetime management. Component object model com is a binaryinterface standard for software components introduced by microsoft in 1993. The name is a bit misleading, in that its not an unknown interface. We pattern our definition for the term interface after the object composition model for reference model of open distributed processing rmodp. Com clients only interact with pointers to interface. Jan 03, 2015 to solve the lifetime part of the problem, its common to use smart interface pointer classes, like ccomptr, which works as far as lifetime is concerned, but we still have to check the hresult, because the smart interface pointer does its magic by implementing t operator const, providing access to the interface. Com is a languageindependent software component model that enables interaction. Iunknown methods can be used to switch between interfaces on an object, add references. In computer programming, the iunknown interface is the fundamental interface in the component object model com. End users of arcgis applications do not necessarily have to understand com, but if you are a developer intent on developing applications based on arcobjects or extending the existing arcgis applications using arcobjects, an understanding of com is a requirement even if you plan to use the.
Therefore, the three methods in iunknown are the first entries in. For debugging purposes, its very useful to know where ano object. Enables clients to get pointers to other interfaces on a given object through the queryinterface method, and manage the existence of the object through the addref and release methods. From this interface definition, the microsoft idl compiler generates. If need to change add methods or arguments a new interface must be defined. If the idispatch interface isnt implemented by the object, you can use the iunknown based com support. 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. To address these questions, it will be helpful to first define the basic com design.
In delphi code, iunknown is simply another name for iinterface. Interface definition is the place at which independent and often unrelated systems meet and act on or communicate with each other. In this way, the iunknown interface is said to define a com objects identity. The following interface would be placed in a vehicle. I have 2 classes, one that must implement iclassfactory and one that must implement iobjectwithsite. All com objects are required to implement the iunknown interface.
Every com object must implement the iunknown interface, also dubbed iinterface in delphi for noncom usage of interfaces as you saw in chapter 2. Software interface synonyms, software interface pronunciation, software interface translation, english dictionary definition of software interface. Com objects specify welldefined interfaces that allow software components to be reused. Interface meaning in the cambridge english dictionary. May 14, 2012 here is the definition of iunknown, defined in objbase. None no class interface will be generated so that com clients need always to go through the interface explicitly defined in our case it is imathctrl. All the data of the interface are hidden from the client, and implemented in the inherited classes of the interface. Introduction to com for arcobjects developers arcobjects.
This means that the three methods defined in iunknown queryinterface, addref, and release are always at the top of any comcompliant vtable, as shown in figure 36. Definition of a software component and its elements. The value returned by createcomobject can then be used to access the methods of the com object after casting it to the desired interface. All methods or api functions that return interface pointers, such as queryinterface, must. Below is an example of when both of these types of interface would be used. It is usually not necessary to call iunknown s methods directly in your. The following image shows the icomponent interface being as a com interface after inheriting from iunknown interface. Microsoft includes the midl compiler with the platform software development kit sdk to enable developers to create the interface definition language idl files and application configuration files acf required for remote procedure call. Every com object must implement at least iunknown, otherwise you simply cant release objects when you no longer need them and calling release is the only way to tell you. Two different developers might define an interface named isum, but so long as both interfaces have different uuids, no confusion will result. An idispatch interface is a special type of iunknown interface that includes methods that allow methods to be called by name at runtime. A software interface allows operators to set up, monitor and audit equipment in seconds, completing processes that used to take significant time, including tracking coin collection, changing price and cycle times on equipment and analyzing service needs.
The reflexive property refers to the ability for the queryinterface call on a given interface with the interfaces id to return the same instance of the interface. As iunknown is a declaration of a com interface, it is an abstract class. Software interfaces a software interface is used to allow either two pieces of software to communicate with each other software software interface, or to allow software to communicate with a hardware device software hardware interface. Arcobjects is based on microsofts component object model com. All com objects support the iunknown interface methods of addref. Interface definition of interface by merriamwebster. A codec property is a keyvalue pair, where the key is a guid and the value is a variant. Thats why this interface is called unknown interface or iunknown. The interpretation of the variant data depends on the property guid.
In computing, an interface is a shared boundary across which two or more separate components of a computer system exchange information. Software interface definition of software interface by. This is the base interface from which every delphi interface inherits, and delphi provides a couple of. Before we begin looking at an example of com development, ill introduce a few com basics. Mark ransom already gave the correct answer any will do, as long as its consistent but picking the first one has one minor advantage. Any derived class must implement the three previously described methods. This function should always return the appropriate interface pointer requested by the caller. Microsoft interface definition language win32 apps. Iunknown interface is the interface from which all other interfaces are derived. Addref increments the reference count, while release decrements it.
The other two iunknown methods, addref and release, manage the reference count. Vba use the createtypelib2 api function to generate the type library when they build their own com components. Iunknown is the base class for all olecomactivex components. Therefore, for example, a com object implemented in visual basic can be.
Software interfaces a software interface is used to allow either two pieces of software to communicate with each other softwaresoftware interface, or to allow software to communicate with a hardware device softwarehardware interface. The iunknown interface is explained fully in the next section. An interface is neither a class nor a com component. Software interface definition of software interface by the. The idl supplied by microsoft is based on simple extensions to dce idl, an industry standard for remote procedure call rpcbased distributed computing. Define an interface ihello using idl language and save the file as hello. The component object model com is a software architecture that allows. Here is an example of the vehicle interface referred to above only a partial definition. To call a method in a simple iunknown interface all you need is to know where in the vtable the function pointer is. This method retrieves the requested interface pointer to the callers. Interfaces are placed in their own files which have the same name as the interface are capitalized and end with the familiar language extension e. He can be contacted at email protected george is a senior computer scientist with developmentor and can be contacted at email protected they are the coauthors of mfc internals addisonwesley, 1996. Descending from iunknown instead of iinterface informs the delphi compiler that the interface must be compatible with com objects. All other com interfaces are inherited, directly or indirectly, from iunknown.
Com defines a protocol that connects one software component, or module, with. The exchange can be between software, computer hardware, peripheral devices, humans, and combinations of these. The iunknown interface is the fundamental interface in com, which contains basic operations for all interfaces and objects. Enables clients to get pointers to other interfaces on a given object through the queryinterface. Com components support a base interface called iunknown described later. To avoid implementing iunknown repeatedly, i created a base class unknown so that for a given child class i only need to implement its direct parent interface. From this interface definition, the microsoft idl compiler generates header files for use by applications using the interface, and source code to handle remote procedure calls. Clients obtain pointers to other interfaces through the iunknown method, queryinterface.
An iunknown or iunknown derived interface consists of a pointer to a virtual method table that contains a list of pointers to the functions that implement the functions declared in the interface, in the order that they are declared in the interface. 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 iunknownaddref and iunknownrelease methods. It is through the iunknown interface that clients maintain a reference on an object while it is in useleaving the actual lifetime management to the object itself. All methods or api functions that return interface pointers, such as queryinterface, must call addref to increment the reference count. The icodecapi interface sets and retrieves settings on an encoder or decoder filter inheritance. For a list of codec property guids, see codec api properties codec profiles. Com component, atl library, com interface, queryinterface. Com is the basis for several other microsoft technologies and frameworks, including ole, ole automation, browser helper object, activex. The microsoft interface definition language midl defines interfaces between client and server programs. The interface definition of iunknown itself is imported from the unknwn.
Feb, 2007 an interface is neither a class nor a com component. This way we have achieved initial goal of iunknown interface. Jun 28, 2010 an idispatch interface is a special type of iunknown interface that includes methods that allow methods to be called by name at runtime. Component object model com is a binary interface standard for software components introduced by microsoft in 1993. The com library contains a set of standard interfaces that define the core.
Iunknown interface is the base interface of every other com interface. Implementing the iunknown interface microsoft docs. Once specification published to world, it is guaranteed not to change, i. Every com object supports at least one interface, the iunknown interface. Queryinterface knows about every interface in the server object and can give a client a pointer to the requested interface. A client that will use this interface will only use a pointer to the interface, and wont need to count on the interface size. Due to layout rules, the iunknown of the first interface will point to the start of the object. Just out of interest, the direct2d interface, id2d1factory inherits from iunknown, so presumably i would still need to implement this interface with. The iunknown interface id is defined as a guid with the value of 0000000000000000c000000000000046 a com components interfaces are required to exhibit the reflexive, symmetric, and transitive properties. Some computer hardware devices, such as a touchscreen, can both send and receive data through the interface, while others such as a mouse or.
Our example in this chapter will develop a complete inprocess component and a matching client. The icodecapi interface inherits from the iunknown interface. To initialize iunknown based com automation objects use createcomobject. Any other iunknown will point to subsequent vtable pointers elsewhere in the object. A com object can be exposed to remote clients via the dcom protocol, in which case it is also a dcom object. In com, a software entity that implements the iunknown interface and zero or more additional interfaces that may be obtained from each other using the iunknown interface. When receiving a pointer to an interface, the client is assured that it can call any method of the interface. It is used to enable interprocess communication object creation in a large range of programming languages.
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