毕业论文5000字英文文献翻译(c++)

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英文翻译

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. 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

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 we've seen already, such as istream and ostream, are all defined as

classesthat is,they are not strictly 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, we'll describe a simple class that

we canuse in solving our bookstore problem. We'll 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 does it support?

For our bookstore problem, we'll 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 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 we've seen, when we use library facilities such as IO, we must include the

associated 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 types 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 aren't picky about header file names, but IDEs

sometimes are. We'll assume that our class is defined in a file named Sales_item.h.

Operations on Sales_item Objects

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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 named

Sales_item. As with the 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" to"aSales_ item object" or even more

simply to "a Sales_item."

In addition to being able to define 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 read a Sales_item object,

Use the output operator, >> to write a Sales_item object,

Use the assignment operator, =, to assign one Sales_item object to another,

Call the same_isbn function 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 -designed class types can be as easy to 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 operations 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 treats 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 technique 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 type's interface; they can think

abstractly 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 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

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

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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 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 types 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 exists 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 data 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."

Applicationsare 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 application.

Authors of successful applications do a good job of understanding and implementing

the needs ofthe application's 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 class 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 that 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 about 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 class. When using the class, the designer shouldn't 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

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is using a class in writing an application. If 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:

internals are protected from inadvertent user-level errors, which might corrupt

the state of the object.

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 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 of places. The data is protected from mistakes that

users might introduce. If there is a bug that corrupts the object's state, the places to look for

the bug are localized: When data are private, only a member function could be responsible

for the error. The search for the mistake is limited, greatly easing the problems of

maintenance and program correctness.

If the data are private and if the interface to the member functions does not change,

then user functions that manipulate class objects require no change.

Because changing a class definition in a header file effectively changes the text of

every source file that includes that header, code that uses a class must be recompiled when

the class changes.

Classes are the most fundamental feature in C++. Classes let us define new types that

are tailored to our own applications, making our programs shorter and easier to modify.

Data abstractionthe ability to define both data and function membersand

encapsulationthe ability to protect class members from general accessare fundamental to

classes. Member functions define the interface to the class. We encapsulate the class by

making the data and functions used by the implementation of a class private.

Classes may define constructors, which are special member functions that control how

objects of the class are initialized. Constructors may be verloaded. Every constructor

should initialize every data member. Constructors should use a constructor initializer list to

initialize the data members. Initializer lists are lists of namevalue pairs where the name is a

member and the value is an initial value for that member.

Classes may grant access to their nonpublic members to other classes or functions. A

class grants access by making the class or function a friend.

Classes may also define mutable or static members. A mutable member is a data

member that is never const; its value may be changed inside a const member function. A

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static member can be either function or data; static members exist independently of the

objects of the class type.

Copy Control

Each type, whether a built-in or class type, defines the meaning of a

set of operations on objects of that type. We can add two int values, run size on a vector,

and so on. These operations define what can be done with objects of the given type.

Each type also defines what happens when objects of the type are created.

Initialization of objects of class type is defined by constructors. Types also control what

happens when objects of the type are copied, assigned, or destroyed. Classes control these

actions through special member functions: the copy constructor, the assignment operator,

and the destructor. This chapter covers these operations.

When we define a new type, we specifyexplicitly or implicitlywhat happens when

objects of that type are copied, assigned, and destroyed. We do so by defining special

members: the copy constructor, the assignment operator, and the destructor. If we do not

explicitly define the copy constructor or the assignment operator, the compiler will

define them for us.

The copy constructor is a special constructor that has a single parameter that is a

reference to the class type. The copy constructor is used explicitly when

we define a new object and initialize it from an object of the same type. It is used implicitly

when we pass or return objects of that type to or from functions.

Collectively, the copy constructor, assignment operator, and destructor are referred to

as copy control. The compiler automatically implements these operations, but the class may

define its own versions.

Copy control is an essential part of defining any C++ class. Programmers new to C++

are often confused by having to define what happens when objects are

copied, assigned, or destroyed. This confusion is compounded because if we do not

explicitly define these operations, the compiler defines them for usalthough they might not

behave as we intend.

Often the compiler-synthesized copy-control functions are finethey do exactly the

work that needs to be done. But for some classes, relying on the default definitions leads to

disaster. Frequently,the most difficult part of implementing the copy-control operations is

recognizing when we need to override the default versions. One especially common case

that requires the class to define its own the copy-control members is if the class has a

pointer member.

The Copy Constructor

The constructor that takes a single parameter that is a reference to an

object of the class type itself is called the copy constructor. Like the default constructor, the

copy constructor can be implicitly invoked by the compiler. The copy constructor is used

to:

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itly or implicitly initialize one object from another of the same type;

an object to pass it as an argument to a function;

an object to return it from a function;

lize the elements in a sequential container;

lize elements in an array from a list of element initializers.

Forms of Object Definition

Recall that C++ supports two forms of initialization