毕业论文5000字英文文献翻译c++.docx

1、毕业论文5000字英文文献翻译c+英文翻译英语原文：. Introducing Classes The only remaining feature we need to understand before solving our bookstore problem is how to write a data structure to represent our transaction data. In C+ we define our own data structure by defining a class. The class mechanism is one of the most

2、 important features in C+. In fact, a primary focus of the design of C+ is to make it possible to define class types that behave as naturally as the built-in types themselves. The library types that weve seen already, such as istream and ostream, are all defined as classesthat is,they are not strict

3、ly speaking part of the language. Complete understanding of the class mechanism requires mastering a lot of information. Fortunately, it is possible to use a class that someone else has written without knowing how to define a class ourselves. In this section, well describe a simple class that we can

4、use in solving our bookstore problem. Well implement this class in the subsequent chapters as we learn more about types,expressions, statements, and functionsall of which are used in defining classes. To use a class we need to know three things: What is its name? Where is it defined? What operations

5、 does it support? For our bookstore problem, well assume that the class is named Sales_item and that it is defined in a header named Sales_item.h. The Sales_item Class The purpose of the Sales_item class is to store an ISBN and keep track of the number of copies sold, the revenue, and average sales

6、price for that book. How these data are stored or computed is not our concern. To use a class, we need not know anything about how it is implemented. Instead, what we need to know is what operations the class provides. As weve seen, when we use library facilities such as IO, we must include the asso

7、ciated headers. Similarly, for our own classes, we must make the definitions associated with the class available to the compiler. We do so in much the same way. Typically, we put the class definition into a file. Any program that wants to use our class must include that file. Conventionally, class t

8、ypes are stored in a file with a name that, like the name of a program source file, has two parts: a file name and a file suffix. Usually the file name is the same as the class defined in the header. The suffix usually is .h, but some programmers use .H, .hpp, or .hxx. Compilers usually arent picky

9、about header file names, but IDEs sometimes are. Well assume that our class is defined in a file named Sales_item.h. Operations on Sales_item Objects Every class defines a type. The type name is the same as the name of the class. Hence, our Sales_item class defines a type namedSales_item. As with th

10、e built-in types, we can define a variable of a class type. When we write Sales_item item we are saying that item is an object of type Sales_item. We often contract the phrase an object of type Sales_item toaSales_ item object or even more simply to a Sales_item. In addition to being able to define

11、variables of type Sales_item, we can perform the following operations on Sales_item objects: Use the addition operator, +, to add two Sales_items, Use the input operator, to write a Sales_item object, Use the assignment operator, =, to assign one Sales_item object to another, Call the same_isbn func

12、tion to determine if two Sales_items refer to the same book. Classes are central to most C+ programs: Classes let us define our own types that are customizedfor the problems we need to solve, resulting in applications that are easier to write and understand.Well-designed class types can be as easy t

13、o use as the built-in types. A class defines data and function members: The data members store the state associated with objectsof the class type, and the functions perform operations that give meaning to the data. Classeslet us separate implementation and interface. The interface specifies the oper

14、ations that the classsupports. Only the implementor of the class need know or care about the details of the implementation. This separation reduces the bookkeeping aspects that make programming tedious anderror-prone. Class types often are referred to as abstract data types. An abstract data type tr

15、eats the data and operations on that state as a single unit. We can think abstractly about what the classd oes, rather than always having to be aware of how the class operates. Abstract data types arefundamental to both object-oriented and generic programming. Data abstraction is a programming techn

16、ique that relies on the separation of interfaceand implementation. The class designer must worry about how a class is implemented, but programmersthat use the class need not know about these details. Instead, programmers who use a type need to know only the types interface; they can think abstractly

17、 about what the type does rather than concretely about how the type works. Encapsulation is a term that describes the technique of combining lower-level elements to forma new, higher-level entity. A function is one form of encapsulation: The detailed actions performedby the function are encapsulated

18、 in the larger entity that is the function itself. Encapsulated elements hide the details of their implementationwe may call a function but have no access to the statements that it executes. In the same way, a class is an encapsulated entity: It represents an aggregation of several members, and most

19、 class types hide the members that implement the type. If we think about the library vector type, it is an example of both data abstraction and encapsulation. It is abstract in that to use it, we think about its interfaceabout the operations that it can perform. It is encapsulated because we have no

20、 access to the details of how the type is representated nor to any of its implementation artifacts. An array, on the other hand, is similar in concept to a vector but is neither abstract nor encapsulated. We manipulate an array directly by accessing the memory in which the array is stored. Not all t

21、ypes need to be abstract. The library pair class is a good example of a useful, well-designed class that is concrete rather than abstract. A concrete class is a class that exposes, rather than hides, its implementation. Some classes, such as pair, really have no abstract interface. The pair type exi

22、sts to bundle two data members into a single object. There is no need or advantage to hiding the data members. Hiding the members in a class like pair would only complicate the use of the type. Even so, such types often have member functions. In particular, it is a good idea for any class that has d

23、ata members of built-in or compound type to define constructor to initialize those members. The user of the class could initialize or assign to the data members but it is less error-prone for the class to do so. Programmers tend to think about the people who will run their applications as users. App

24、licationsare designed for and evolve in response to feedback from those who ultimately use the applications. Classes are thought of in a similar way: A class designer designs and implements a class for users of that class. In this case, the user is a programmer, not the ultimate user of the applicat

25、ion. Authors of successful applications do a good job of understanding and implementing the needs ofthe applications users. Similarly, well-designed, useful classes are designed with a close attention to the needs of the users of the class. In another way, the division between class designer and cla

26、ss user reflects the division betweenusers of an application and the designers and implementors of the application. Users care only if the application meets their needs in a cost-effective way. Similarly, users of a class care only about its interface. Good class designers define a class interface t

27、hat is intuitive and easy to use. Users care about the implementation only in so far as the implementation affects their use of the class. If the implementation is too slow or puts burdens on users of the class, then the users must care. In well-designed classes, only the class designer worries abou

28、t the implementation. In simple applications, the user of a class and the designer of the class might be one and the same person. Even in such cases, it is useful to keep the roles distinct. When designing the interface to a class, the class designer should think about how easy it will be to use the

29、 class. When using the class, the designer shouldnt think about how the class works. When referring to a user, the context makes it clear which kind of user is meant. If we speak of user code or the user of the Sales_item class, we mean a programmer who is using a class in writing an application. If

30、 we speak of the user of the bookstore application, we mean the manager of the store who is running the application. Data abstraction and encapsulation provide two important advantages: 1.Class internals are protected from inadvertent user-level errors, which might corrupt the state of the object. 2

31、.The class implementation may evolve over time in response to changing requirements or bug reports without requiring change in user-level code. By defining data members only in the private section of the class, the class author is free to make changes in the data. If the implementation changes, only

32、 the class code needs to be examined to see what affect the change may have. If data are public, then any function that directly accesses the data members of the old representation might be broken. It would be necessary to locate and rewrite all those portions of code that relied on the old pesentation before the program could be used again. Similarly, if the internal state of the class is private, then changes to the member data can happen in only a limited number o