使用C语言实现面向对象编程

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"这份PDF文档名为《用C实现面向对象》(英文版)，主要探讨了如何在C语言这种非原生支持面向对象特性的编程语言中实现面向对象编程的概念和技术。文档的前言指出，没有任何一种编程技术能解决所有问题，也没有任何一种编程语言能确保程序绝对正确。面向对象编程（OOP）被广泛视为万能解决方案，尽管它已经存在超过十年。作者强调，不应该每个项目都从零开始，而OOP的核心是应用长期以来的优秀编程原则。 C++等语言被视为新语言，因为它们支持面向对象特性，但即使不使用这些特性，也可以用纯ANSI C实现相同的效果。面向对象的主要优点在于代码重用，这并非新概念，程序员们长期以来都有自己的工具库和函数库。这本书不是为了推崇或贬低面向对象编程，而是通过ANSI C来探索OOP的实现方式、所涉及的技术以及为何这些技术有助于解决问题和提高软件设计的效率。书中可能涵盖了以下关键知识点： 1. **封装**：面向对象中的核心概念之一，通过数据隐藏和接口定义，将数据和操作数据的方法封装在一起，以保护数据免受外部的不当访问。 2. **继承**：允许创建一个类作为另一个类的子类，子类可以继承父类的属性和方法，减少了代码重复，并且提供了多态的基础。 3. **多态**：指同一种行为在不同对象上有不同的表现形式，这在C中可以通过函数指针和结构体结合的方式实现。 4. **抽象类与接口**：虽然C语言没有内置的抽象类或接口机制，但可以通过模拟实现，例如定义一组共用的函数原型，让多个类实现这些函数来达到类似的效果。 5. **消息传递**：面向对象中的对象间通信方式，C语言中通常通过函数调用来模拟。 6. **动态绑定**：在运行时确定对象方法的调用，C语言中可以通过函数指针实现。 7. **设计模式**：在C中实现面向对象时，可能会介绍一些经典的设计模式，如工厂模式、单例模式、观察者模式等，这些模式有助于提高代码的可读性和可维护性。 8. **模拟类与对象**：C语言中没有内置的类机制，但可以通过结构体和指向结构体的指针模拟类的行为，通过结构体实例来模拟对象。 9. **代码重用**：通过宏定义、函数库、对象模型等方式在C中实现代码重用，以提高开发效率。 10. **案例研究**：书中可能包含实际的C代码示例，展示如何在实际项目中应用面向对象的思想。通过学习这本书，读者将能够理解如何在不支持OOP特性的语言中利用面向对象原则进行编程，这对于理解面向对象的本质和提升C语言编程能力都非常有帮助。"

___________________________________________________________________________

1 Abstract Data Types

— Information Hiding

apply() calls action() for each element in set and passes the rest of the argument

list. action() must not change set but it may return zero to terminate apply() early.

apply() returns true if all elements were processed.

___________________________________________________________________________

Dynamic Linkage

Generic Functions

2.1 Constructors and Destructors

Let us implement a simple string data type which we will later include into a set.

For a new string we allocate a dynamic buffer to hold the text. When the string is

deleted, we will have to reclaim the buffer.

new() is responsible for creating an object and delete() must reclaim the

resources it owns. new() knows what kind of object it is creating, because it has

the description of the object as a first parameter. Based on the parameter, we

could use a chain of if statements to handle each creation individually. The draw-

back is that new() would explicitly contain code for each data type which we sup-

port.

delete(), however, has a bigger problem. It, too, must behave differently based

on the type of the object being deleted: for a string the text buffer must be freed;

for an object as used in chapter 1 only the object itself has to be reclaimed; and a

set may have acquired various chunks of memory to store references to its ele-

ments.

We could give delete() another parameter: either our type descriptor or the

function to do the cleaning up, but this approach is clumsy and error-prone. There

is a much more general and elegant way: each object must know how to destroy

its own resources. Part of each and every object will be a pointer with which we

can locate a clean-up function. We call such a function a destructor for the object.

Now new() has a problem. It is responsible for creating objects and returning

pointers that can be passed to delete(), i.e., new() must install the destructor infor-

mation in each object. The obvious approach is to make a pointer to the destructor

part of the type descriptor which is passed to new(). So far we need something like

the following declarations:

struct type {

size_t size; /* size of an object */

void (* dtor) (void *); /* destructor */

};

struct String {

char * text; /* dynamic string */

const void * destroy; /* locate destructor */

};

struct Set {

... information ...

const void * destroy; /* locate destructor */

};

___________________________________________________________________________

2 Dynamic Linkage — Generic Functions

It looks like we have another problem: somebody needs to copy the destructor

pointer dtor from the type description to destroy in the new object and the copy

may have to be placed into a different position in each class of objects.

Initialization is part of the job of

new() and different types require different work

— new() may even require different arguments for different types:

new(Set); /* make a set */

new(String, "text"); /* make a string */

For initialization we use another type-specific function which we will call a

construc-

tor. Since constructor and destructor are type-specific and do not change, we pass

both to new() as part of the type description.

Note that constructor and destructor are not responsible for acquiring and

releasing the memory for an object itself — this is the job of new() and delete().

The constructor is called by new() and is only responsible for initializing the memory

area allocated by new(). For a string, this does involve acquiring another piece of

memory to store the text, but the space for struct String itself is allocated by

new(). This space is later freed by delete(). First, however, delete() calls the des-

tructor which essentially reverses the initialization done by the constructor before

delete() recycles the memory area allocated by new().

2.2 Methods, Messages, Classes and Objects

delete() must be able to locate the destructor without knowing what type of object

it has been given. Therefore, revising the declarations shown in section 2.1, we

must insist that the pointer used to locate the destructor must be at the beginning

of all objects passed to delete(), no matter what type they have.

What should this pointer point to? If all we have is the address of an object,

this pointer gives us access to type-specific information for the object, such as its

destructor function. It seems likely that we will soon invent other type-specific

functions such as a function to display objects, or our comparison function differ(),

or a function clone() to create a complete copy of an object. Therefore we will use

a pointer to a table of function pointers.

Looking closely, we realize that this table must be part of the type description

passed to new(), and the obvious solution is to let an object point to the entire type

description:

struct Class {

size_t size;

void * (* ctor) (void * self, va_list * app);

void * (* dtor) (void * self);

void * (* clone) (const void * self);

int (* differ) (const void * self, const void * b);

};

struct String {

const void * class; /* must be first */

char * text;

};

___________________________________________________________________________

2.3 Selectors, Dynamic Linkage, and Polymorphisms

struct Set {

const void * class; /* must be first */

...

};

Each of our objects starts with a pointer to its own type description, and through

this type description we can locate type-specific information for the object:

.size is

the length that new() allocates for the object; .ctor points to the constructor called

by new() which receives the allocated area and the rest of the argument list passed

to new() originally; .dtor points to the destructor called by delete() which receives

the object to be destroyed; .clone points to a copy function which receives the

object to be copied; and .differ points to a function which compares its object to

something else.

Looking down this list, we notice that every function works for the object

through which it will be selected. Only the constructor may have to cope with a

partially initialized memory area. We call these functions methods for the objects.

Calling a method is termed a message and we have marked the receiving object of

the message with the parameter name self. Since we are using plain C functions,

self need not be the first parameter.

Many objects will share the same type descriptor, i.e., they need the same

amount of memory and the same methods can be applied to them. We call all

objects with the same type descriptor a class; a single object is called an instance

of the class. So far a class, an abstract data type, and a set of possible values

together with operations, i.e., a data type, are pretty much the same.

An object is an instance of a class, i.e., it has a state represented by the

memory allocated by new() and the state is manipulated with the methods of its

class. Conventionally speaking, an object is a value of a particular data type.

2.3 Selectors, Dynamic Linkage, and Polymorphisms

Who does the messaging? The constructor is called by new() for a new memory

area which is mostly uninitialized:

void * new (const void * _class, ...)

{ const struct Class * class = _class;

void * p = calloc(1, class —> size);

assert(p);

* (const struct Class **) p = class;

if (class —> ctor)

{ va_list ap;

va_start(ap, _class);

p = class —> ctor(p, & ap);

va_end(ap);

}

return p;

}

The existence of the struct Class pointer at the beginning of an object is extremely

important. This is why we initialize this pointer already in new():

___________________________________________________________________________

2 Dynamic Linkage

— Generic Functions

•p

object

•

...........

class

size

ctor

dtor

clone

differ

struct Class

The type description class at the right is initialized at compile time. The object is

created at run time and the dashed pointers are then inserted. In the assignment

* (const struct Class **) p = class;

p points to the beginning of the new memory area for the object. We force a

conversion of p which treats the beginning of the object as a pointer to a struct

Class and set the argument class as the value of this pointer.

Next, if a constructor is part of the type description, we call it and return its

result as the result of new(), i.e., as the new object. Section 2.6 illustrates that a

clever constructor can, therefore, decide on its own memory management.

Note that only explicitly visible functions like new() can have a variable parame-

ter list. The list is accessed with a va_list variable ap which is initialized using the

macro va_start() from stdarg.h. new() can only pass the entire list to the construc-

tor; therefore, .ctor is declared with a va_list parameter and not with its own vari-

able parameter list. Since we might later want to share the original parameters

among several functions, we pass the address of ap to the constructor — when it

returns, ap will point to the first argument not consumed by the constructor.

delete() assumes that each object, i.e., each non-null pointer, points to a type

description. This is used to call the destructor if any exists. Here, self plays the

role of p in the previous picture. We force the conversion using a local variable cp

and very carefully thread our way from self to its description:

void delete (void * self)

{ const struct Class ** cp = self;

if (self && * cp && (* cp) —> dtor)

self = (* cp) —> dtor(self);

free(self);

}

The destructor, too, gets a chance to substitute its own pointer to be passed to

free() by delete(). If the constructor decides to cheat, the destructor thus has a

chance to correct things, see section 2.6. If an object does not want to be deleted,

its destructor would return a null pointer.

All other methods stored in the type description are called in a similar fashion.

In each case we have a single receiving object self and we need to route the

method call through its descriptor:

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