ANSI-C探索面向对象编程：实践与理解

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"《ANSI-C实现面向对象》是一本深入探讨面向对象编程在C语言中的应用与实践的书籍。作者强调尽管C++、Java等现代语言因其面向对象特性而备受推崇，但其实ANSI-C也能有效地实现这一设计理念，尤其是在项目复用和模块化方面。本书的核心观点并不在于推崇或否定某一特定编程范式，而是通过实际操作，用ANSI-C揭示面向对象编程的本质。书中的主要内容包括对面向对象编程基础概念的介绍，如封装、继承和多态等核心原则。作者指出，尽管C++是公认的“新语言”因其全面的面向对象支持，但这并不意味着非面向对象的C就不能实现类似的功能。实际上，即使在ANSI-C中，程序员也可以通过精心设计类和结构体来模拟对象的行为，实现代码的重用。在本书中，读者可以学习到如何在C语言环境中运用面向对象的设计思想，理解这些技术为何能帮助解决复杂的问题。作者可能会详细阐述如何在C中创建类、构造函数、析构函数以及如何通过指针和结构体实现接口的抽象，这些都是面向对象编程的关键要素。此外，书中可能还会讨论如何避免在面向对象编程中常见的陷阱，比如过度设计、不必要的耦合和性能开销。通过比较和分析，读者将能够更好地理解面向对象编程的优势和局限性，从而在实际项目中做出更明智的选择。《ANSI-C实现面向对象》是一本适合C语言开发者提升技能，扩展编程视野的实用教程，它不仅教授技术知识，还引导读者思考如何在不同编程语言背景下灵活运用面向对象的设计方法。无论是想要深入C语言的开发者，还是希望跨越语言障碍理解面向对象的程序员，这本书都将提供有价值的见解和实践经验。"

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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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