C语言中的面向对象编程：状态机与C库应用

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本资源是一份名为《AN_Simple_OOP_in_C.pdf》的应用笔记，主要探讨了如何在C语言中应用面向对象编程（Object-Oriented Programming, OOP）的概念，特别是针对状态机（State Machine, SM）中的QP（QuantumLeaps）方法。这份文档由QuantumLeaps, LLC发布，版权日期为2017年10月，强调了C语言中的封装、继承、多态性（包括虚拟函数）、虚表（virtual table, vtbl）和虚指针（virtual pointer, vptr）等关键概念。 1. **介绍**：文档首先介绍了面向对象编程的基本理念，旨在使读者理解如何在C语言这种传统的低级语言中实现面向对象设计。 2. **封装**：章节中详细解释了封装的重要性，即隐藏对象的内部细节，只暴露必要的接口，以便于代码的复用和维护。 3. **继承**：C语言并不原生支持类继承，但通过结构体和指针可以模拟，文档讲解了如何创建子类并继承父类的属性和行为。 4. **多态性与虚拟函数**： - **虚拟表（vtbl）和虚拟指针（vptr）**：虚拟表是一种数据结构，存储了指向每个基类虚函数的实际地址；而虚指针是在派生类对象中用来指向其父类vtbl的指针。 - **设置vptr**：在构造函数中设置vptr，确保正确指向子类的虚函数。 - **继承vtbl和重写vptr**：子类继承父类的vtbl，并可能在其内部重定义某些函数，以实现不同的行为。 - **虚拟调用（late binding）**：通过vptr进行的函数调用，在运行时确定实际调用哪个函数，实现了多态。 5. **使用虚拟函数的例子**：文档提供了实际示例，展示了如何在C语言中通过虚拟函数来实现面向对象设计中的灵活性。 6. **总结**：文档总结了面向对象编程在C语言中的核心要点，强调了它对复杂系统管理的优势。 7. **参考文献**：文档提供了进一步阅读的参考资料，帮助读者深入学习相关主题。 8. **联系信息**：文档包含了QuantumLeaps的联系方式，以便用户查询或提供反馈。 9. **法律免责声明**：最后，文档强调了信息的准确性，但使用者需自行判断，QuantumLeaps不对信息的完整性和使用后果负责，并保留对文档内容进行更新的权利。这份资源对于希望在C语言中引入面向对象编程思想，尤其是处理状态机问题的开发人员来说，具有很高的实用价值。通过阅读和实践，开发者将能够更好地理解和利用这些技术来提升代码的可维护性和扩展性。

1 Introduction

Object-oriented programming (OOP) is not the use of a particular language or a tool. It is rather a way of

design based on the three fundamental design meta-patterns:

• Encapsulation – the ability to package data and functions together into classes

• Inheritance – the ability to define new classes based on existing classes in order to obtain reuse and

code organization

• Polymorphism – the ability to substitute objects of matching interfaces for one another at run-time

Although these meta-patterns have been traditionally associated with object-oriented languages, such as

Smalltalk, C++, or Java, you can implement them in almost any programming language including portable

ANSI-C

[1,2,3,4,5,6]

NOTES: If you simply develop end-user programs in C, but you also want to do OOP, you probably

should be using C++ instead of C. Compared to C++, OOP in C can be cumbersome and error-

prone, and rarely offers any performance advantage.

However, if you build software libraries or frameworks the OOP concepts can be very useful as the

primary mechanisms of organizing the code. In that case, most difficulties of doing OOP in C can be

confined to the library and can be effectively hidden from the application developers. This document

has this primary use case in mind.

This Application Note describes how OOP is implemented in the QP/C and QP-nano real-time

frameworks. As a user of these frameworks, you need to understand the techniques, because you

will need to apply them also to your own application-level code. But these techniques are not limited

only to developing QP/C or QP-nano applications and are applicable generally to any C program.

2 Encapsulation

Encapsulation is the ability to package data with functions into classes. This concept should actually

come as very familiar to any C programmer because it’s quite often used even in the traditional C. For

example, in the Standard C runtime library, the family of functions that includes

fopen()

fclose()

fread()

fwrite()

, etc. operates on objects of type

FILE

. The

FILE

structure is thus encapsulated

because client programmers have no need to access the internal attributes of the

FILE struct

and

instead the whole interface to files consists only of the aforementioned functions. You can think of the

FILE

structure and the associated C-functions that operate on it as the

FILE

class. The following bullet

items summarize how the C runtime library implements the

FILE

class:

1. Attributes of the class are defined with a C

struct

(the

FILE struct

2. Operations of the class are defined as C functions. Each function takes a pointer to the attribute

structure (

FILE *

) as an argument. Class operations typically follow a common naming convention

(e.g., all

FILE

class methods start with prefix

3. Special functions initialize and clean up the attribute structure (

fopen()

and

fclose()

). These

functions play the roles of class constructor and destructor, respectively.

You can very easily apply these design principles to come up with your own “classes”. For example,

suppose you have an application that employs two-dimensional geometric shapes (perhaps to be

rendered on an embedded graphic LCD). The basic

Shape

“class” in C can be declared as follows:

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