ANSI-C实现面向对象编程：从原理到实践

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"面向对象编程（Object-Oriented Programming, OOP）是现代软件开发中的一种核心理念，特别是在C++、Java和Python等高级编程语言中占据重要地位。本文提到的《Object-Oriented Programming With ANSI-C》是一本旨在用C语言来探索和理解OOP概念的小册子，因为尽管C++以其面向对象特性而闻名，但其实C语言本身也可以实现这些原则，只是可能需要更为巧妙的设计。该书的核心观点是，虽然没有一种编程技术能解决所有问题，也没有一种语言能保证结果总是正确，但OOP提供了一种组织代码和解决问题的新方法。它强调的是良好的编程原则，如封装（Encapsulation）、继承（Inheritance）和多态（Polymorphism），这些都是OOP语言中的关键特性： 1. 封装：在面向对象中，数据和操作数据的方法被封装在一起，形成类（Class）。这样可以隐藏内部实现细节，仅对外提供接口，增强了代码的可维护性和安全性。 2. 继承：子类（Child Class）可以从父类（Parent Class）继承属性和行为，减少了代码重复，提高了复用性。这是实现“代码重用”（Code Reuse）的关键手段。 3. 实例和连接：对象是类的实例，它们之间可以通过指针或引用进行连接，实现交互。在C中，虽然不像其他OOP语言那样直观，但可以通过结构体和函数指针来模拟类似的概念。 4. 方法：方法是封装的体现，它们是与类关联的函数，处理特定任务。在C中，通过函数指针或者结构体成员函数来实现类似功能。 5. 多态：多态允许同一种操作在不同类型的对象上产生不同的效果。在C中，尽管不直接支持，但通过函数指针或结构体的动态类型转换可以间接实现某种程度的动态多态。 6. 项目重用与模块化：OOP强调模块化设计，允许在不同项目中重用代码，减少工作量。C程序员可以创建库或模块，提高代码的复用性。 7. 问题解决策略：通过OOP，程序员可以更好地组织代码，使得程序结构清晰，更容易发现并修复错误。它促进了抽象思考，帮助我们解决更复杂的问题。 8. 不断学习和进化：作者指出，尽管C++被视为新语言，但并不是因为它的面向对象特性，而是因为它提供了新的编程范式。实际上，即使在ANSI-C这样的基础语言中，通过适当的设计和利用现有工具，也能实现很多OOP的效果。《Object-Oriented Programming With ANSI-C》是一本实用的教程，它鼓励读者通过C语言深入理解面向对象编程的核心概念，并教会读者如何在不使用特定OOP语言的情况下，通过C来实现类似的功能，从而提升编程技能和解决问题的能力。书中并未局限于对或错的评判，而是以实践为导向，帮助开发者掌握面向对象编程的精髓。"

___________________________________________________________________________

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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