Component Object Model: COM

I. Components

(Com) is a new software development technology developed by Microsoft to make software production in the computer industry more in line with human behavior. Under the com architecture, people can develop a variety of unique functional components and then combine them as needed to form a complex application system. The benefits are as follows: new components in the system can be replaced to upgrade and customize the system at any time; the same component can be reused in multiple application systems; the application system can be easily extended to the network environment; com and language, platform-independent features allow all programmers to make full use of their talents and expertise to write component modules.

Com is a method for developing software components. Components are actually some small binary executable programs that can provide services to applications, operating systems, and other components. Developing custom com components is like developing dynamic and object-oriented APIs. Multiple com objects can be connected to form an application or component system. In addition, components can be removed or replaced at runtime without being relinked or compiled. Many microsoft technologies, such as activex, directx, and ole, are built based on com. Microsoft developers also use com components to customize their applications and operating systems.

The concept of com is not only effective in microsoft windows. Com is not a big api. It is actually a programming method like structured programming and object-oriented programming. In any operating system, developers can follow the "com method ".

An application usually consists of a single binary file. After the compiler generates an application, the application will not change until the next version is re-compiled and the new version is released. Changes to the operating system, hardware, and customer needs must be generated after the entire application is regenerated.

This situation has changed. Developers start to separate a single application into multiple independent components. The advantage of this approach is that new components can be used to replace existing components with the continuous development of technology. At this time, applications can gradually improve as new components constantly replace old ones. In addition, you can quickly create new applications by using existing components.

The traditional approach is to split applications into files, modules, or classes, and then compile and link them into a single-mode application. It is very different from the process in which components build applications (called component architecture. A component is similar to a micro-application, that is, binary code that has been compiled and linked and can be used. An application is packaged by multiple such components. A single-mode application has only one binary code module. Custom components can be connected to other components at runtime to form an application. To modify or improve an application, you only need to replace one of the components that constitute the application with a new version.

Com, a component object model, describes how to create components and how to use components to create applications.

Advantages of using components:

One advantage of component architecture is that applications can evolve over time. In addition, you can also use components to make it easier and more flexible to upgrade applications, such as application customization, Component Libraries, and distributed components.

The advantages of using components directly come from the ability to dynamically insert or unload them from applications. To implement this function, all components must meet two conditions: first, components must be dynamically linked; second, they must be hidden (or encapsulated) internal implementation details. Dynamic Links are a crucial requirement for components, while message hiding is a necessary condition for dynamic links.

Ii. Interface

For com, the interface is a memory structure that contains an array of function pointers. Each array element contains a function address implemented by the component. For com, the interface is the memory structure, and other things are the implementation details that com does not care about.

In c ++, abstract base classes can be used to implement com interfaces. Since a com component can support any number of interfaces, such components can be implemented using multiple inheritance of abstract base classes. It is easier to use classes to implement components than other methods.

For customers, a component is an interface set. The customer can only deal with com components through interfaces. On the whole, the customer knows little about a component. Generally, you do not even need to know all interfaces provided by a component.

The customer interacts with the component through the interface. Other interfaces of the customer query component are also completed through interfaces. This interface is iunknown. The definition of the iunknown interface is included in the header file of unknown. h in win32 sdk. The reference is as follows:

Interface iunknown

{

Virtual hresult-_ stdcall queryinterface (const iid & iid, void ** GMM) = 0;

Virtual ulong _ stdcall addref () = 0;

Virtual ulong _ release () = 0;

};

All com must inherit iunknown. You can use the iunknown interface pointer to query other interfaces of the component. The first three functions of each interface in vtbl are queryinterface, addref, and release. This allows all com interfaces to be processed as iunknown interfaces. Because all interfaces support queryinterface, any interface of the component can be used by the customer to obtain other interfaces supported by the component.

After using queryinterface to abstract a component into a set composed of multiple independent interfaces, you also need to manage the life cycle of the component. This is achieved through the reference count of the interface. The customer cannot directly control the life cycle of components. When one interface is used but another interface of the component is used, the modified component cannot be released. The release of components can be completed by the customer after all the components are used. The other two member functions addref and release of iunknown are used to provide customers with a means to instruct them when to process an interface.

Addref and release implement a memory management technology named reference technology. When the customer obtains an interface from the component, the reference count value increases by 1. When a customer uses an interface, the reference count of the component is reduced by 1. When the reference count is 0, the component can delete itself from the memory. Addref and release can increase or decrease this Count value.

3. Create

Splitting a component into multiple interfaces is only the first step in dividing a single-mode application into multiple parts. The component must be placed in a dynamic link library (dll. Dll is a component service program, or a method for distributing components. The component should be regarded as the interface set implemented in the dll. Before a customer obtains a component interface pointer, it must first load the corresponding dll into its process space and create the component.

Because all functions required by the customer component can be accessed through an interface pointer, you can introduce the creatinstance function in the dll so that users can call it. Then, you can load the dll and call the functions in it. This function can be implemented by the com library function cocreateinstance. Cocreateinstance creates a component by passing it a clsid, then it creates the corresponding component, and returns a pointer to the requested interface. However, cocreateinstance does not provide customers with a method to control the component creation process and lacks some flexibility. In fact, common class factories are used to create components. A class factory is a component with an interface for creating other components. The customer first creates the class factory itself, and then uses an interface (such as iclassfactory) to create the required components. Then, use dllregistersever to register this component in windows.

Iv. Reuse

Com components can be reused and support "interface inheritance ". This Inheritance refers to the type or interface that a class inherits from its base class. Abstract base classes are the purest inheritance of interfaces and are also used to implement com interfaces. In com, we can use inclusion and aggregation to transform components.

Inclusiveness is completed at the interface level. An external component contains a pointer to an internal interface. In this case, the external component is only a customer of the internal component. It uses the interface of the internal component to implement its own interface. External components can also re-implement an interface supported by internal components by forwarding calls to internal components. In addition, external components can add some code before and after the internal component code to transform the interface.

Aggregation is a form of change. When an external component aggregates an interface of an internal component, it does not implement this interface again as it is inclusive and explicitly forwards call requests to the internal component. On the contrary, the external component directly returns the interface pointer of the internal component to the customer. Using this method, external components do not need to implement and forward all functions in the interface.

Inclusiveness and convergence provide a robust mechanism for Component reuse. In the component architecture, the customer completely isolated the component implementation.

V. Summary

The above are some basic knowledge about com. Components compiled according to the com specification will greatly change the traditional software production method and have broad development prospects. This will also introduce new content and methods for software engineering.

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