使用C语言实现面向对象编程
使用C语言实现面向对象编程
使用C语言实现面向对象编程 – tommwq.tech/blog
面向对象是一种程序设计方法。面向对象不是某种语法或语言特性，因此使用任何高级语言都可以实现面向对象程序设计。与之相似的，使用面向对象程序语言，也可以做出非面向对象的程序设计。这里简单介绍一种用C语言实现面向对象的方法。
面向对象的核心原则是使用对象来组织程序。对象是可以执行某些行为的东西。为了保证行为是正确的，对象需要维护控制行为的一组状态。要避免状态被外部代码破坏，对象必须保护这些状态，这就产生了面向对象的第一个特性：封装。
在C语言中，结构体可以将一组相关的状态（变量）保存在一起。这是否就是封装呢？答案是否定的。结构体只是状态的简单聚合，无法起到保护状态的作用。封装的目的在于保护状态的一致性，必须禁止外部代码直接修改状态。因此我们需要使用结构体和函数相结合的方法来实现封装。这里要注意，为了避免客户代码直接修改结构体，需要将结构体定义保存在私有的.c文件中。头文件保留前向声明，在函数中使用结构体指针。下面看一个例子。
Listing 1: persion.h
struct Person; struct Person* Person_create(); void Person_destroy(struct Person*); int Person_set_age(struct Person *person, unsigned int age); int Person_set_name(struct Person *person, const char *name);
Listing 2: person.c
#define MAX_NAME_LENGTH 64
#define MAX_AGE 120
struct Person {char name[MAX_NAME_LENGTH];unsigned int age;
};int Person_set_age(struct Person *person, unsigned int age) {if (age > MAX_AGE) {return -1;}person->age = age;return 0;
}int Person_set_name(struct Person *person, const char *name) {if (strlen(name) > MAX_NAME_LENGTH) {return -1;}strncpy(person->name, name);return 0;
}
上面的代码实现了一个简单的业务模型：一个人有名字和年龄。这个模型有两个约束：
- 名字长度不超过MAX_NAME_LENGTH。
- 年龄不超过MAX_AGE。
如果把结构体Person直接暴露给客户代码，我们将无法保证对象状态始终是符合约束的。因为客户代码可能会设置一个错误的状态，比如：
struct Person *person = Person_create(); person->age = MAX_AGE + 100; // 状态约束被客户代码破坏。
而通过隐藏结构体定义和定义操作函数的方法，我们可以在操作函数中对状态的转移进行控制，保证状态始终是满足约束的，是合法的。
面向对象的第二个特性是继承：子类可以继承父类的状态和行为。继承状态可以通过将父类结构体包含在子类中实现。为了让子类继承父类行为，我们将父类的操作保存到一个接口结构体中，通过复制这个接口结构体实现行为的继承。父类需要作为子类的第一个成员，这样通过指针引用时，子类实例和父类实例可以使用同一个指针表示。
typedef void (*Student_print_name)(struct Student *);
typedef void (*Student_set_name)(struct Student *, const char *);
typedef void (*Student_print_score)(struct Student *s);
typedef void (*Student_set_score)(struct Student *s, int score);
typedef int (*Student_get_score)(struct Student *s);
typedef int (*Student_compare_score)(struct Student *lhs, struct Student *rhs);
typedef void (*Student_destroy)(struct Student *s);struct StudentInterface {Student_print_name print_name;Student_set_name set_name;Student_print_score print_score;Student_set_score set_score;Student_get_score get_score;Student_compare_score compare_score;Student_destroy destroy;
};
struct StudentInterface _student_interface;struct Object {void* interface;
};struct Student {struct Object object;char name[16];int score;
};struct CheatStudent {struct Student base;int cheated;
};struct CheatStudentInterface _cheatstudent_interface;void CheatStudent_class_init() {// 通过复制父类接口列表，实现行为继承。memcpy(&_cheatstudent_interface.base, _student_interface, sizeof(_student_interface));
}struct CheatStudent* CheatStudent_create() {struct CheatStudent *s = (struct CheatStudent*) malloc(sizeof(struct CheatStudent));if (s == NULL) {return NULL;}s->base.object.interface = &_cheatstudent_interface;s->cheated = 0;return s;
}
面向对象的另一个特性是多态。为了实现多态，我们需要根据实例的具体类型进行函数分派。我们的做法是将函数分派表保存到类实例中。每个类拥有自己的函数表。在初始化类的时候设置这个函数表。同一个类的实例共享这个函数表。每个实例中都包含一个指针指向类函数表。函数表也使用结构体表示。父类函数表是子类的函数表的第一个成员。在初始化子类（不是初始化子类实例）时，将父类的函数表复制到子类函数表起始的位置。然后初始化子类的特有函数。在分派函数时，根据实例内的指针找到函数表，然后根据函数表进行分派。
Listing 3: student.h
struct Student;typedef void (*Student_print_name)(struct Student *);
typedef void (*Student_set_name)(struct Student *, const char *);
typedef void (*Student_print_score)(struct Student *s);
typedef void (*Student_set_score)(struct Student *s, int score);
typedef int (*Student_get_score)(struct Student *s);
typedef int (*Student_compare_score)(struct Student *lhs, struct Student *rhs);
typedef void (*Student_destroy)(struct Student *s);struct StudentInterface {Student_print_name print_name;Student_set_name set_name;Student_print_score print_score;Student_set_score set_score;Student_get_score get_score;Student_compare_score compare_score;Student_destroy destroy;
};typedef void (*CheatStudent_cheat)(struct CheatStudent *cs);struct CheatStudentInterface {struct StudentInterface base;CheatStudent_cheat cheat;
};void Student_class_init();
struct Student* Student_create();struct CheatStudent;
void CheatStudent_class_init();
struct CheatStudent* CheatStudent_create();#define AS_INTERFACE(INTERFACE, POINTER_TO_INSTANCE) ((INTERFACE *) *((void**)POINTER_TO_INSTANCE))
#define CREATE_DERIVED_INSTANCE(BASE_TYPE, CREATOR) (BASE_TYPE) (CREATOR())
Listing 4: student.c
#include#include #include "student.h"struct Object {void* interface; };struct Student {struct Object object;char name[16];int score; };struct StudentInterface _student_interface;void _student_print_name(struct Student *s) {printf("%s\n", s->name); }void _student_set_name(struct Student *s, const char *name) {strncpy(s->name, name, 16); }void _student_print_score(struct Student *s) {printf("%d\n", s->score); }void _student_set_score(struct Student *s, int score) {s->score = score; }int _student_get_score(struct Student *s) {return s->score; }int _student_compare_score(struct Student *lhs, struct Student *rhs) {int a = lhs->score;int b = rhs->score;if (a < b) {return -1;} else if (a == b) {return 0;} else {return 1;} }void _student_destroy(struct Student *student) {free(student); }struct Student* Student_create() {struct Student *s = (struct Student*) malloc(sizeof(struct Student));if (s == NULL) {return NULL;}s->object.interface = &_student_interface;return s; }void Student_class_init() {_student_interface.print_name = _student_print_name;_student_interface.print_score = _student_print_score;_student_interface.set_name = _student_set_name;_student_interface.set_score = _student_set_score;_student_interface.get_score = _student_get_score;_student_interface.compare_score = _student_compare_score;_student_interface.destroy = _student_destroy; }struct CheatStudent {struct Student base;int cheated; };void _cheat_student_cheat(struct CheatStudent* cs) {AS_INTERFACE(struct StudentInterface, cs)->set_score(cs, 100);cs->cheated = 1; }struct CheatStudentInterface _cheatstudent_interface;struct CheatStudent* CheatStudent_create() {struct CheatStudent *s = (struct CheatStudent*) malloc(sizeof(struct CheatStudent));if (s == NULL) {return NULL;}s->base.object.interface = &_cheatstudent_interface;s->cheated = 0;return s; }void _cheatstudent_print_score(struct CheatStudent *s) {int score = AS_INTERFACE(struct StudentInterface, &(s->base))->get_score(&(s->base));printf("%d%s\n", score, s->cheated ? "(cheat)" : ""); }void CheatStudent_class_init() {memcpy(&_cheatstudent_interface.base, _student_interface, sizeof(_student_interface));_cheatstudent_interface.base.print_score = _cheatstudent_print_score;_cheatstudent_interface.cheat = _cheat_student_cheat; }
Listing 5: main.c
#include#include #include "student.h"int main() {Student_class_init();struct Student *student = Student_create();AS_INTERFACE(struct StudentInterface, student)->set_name(student, "Alice");AS_INTERFACE(struct StudentInterface, student)->print_name(student);AS_INTERFACE(struct StudentInterface, student)->set_score(student, 90);AS_INTERFACE(struct StudentInterface, student)->print_score(student);AS_INTERFACE(struct StudentInterface, student)->destroy(student);CheatStudent_class_init();struct CheatStudent *cheat_student = CREATE_DERIVED_INSTANCE(struct Student*, CheatStudent_create);AS_INTERFACE(struct StudentInterface, cheat_student)->set_name(cheat_student, "Bob");AS_INTERFACE(struct StudentInterface, cheat_student)->print_name(cheat_student);AS_INTERFACE(struct StudentInterface, cheat_student)->set_score(cheat_student, 90);AS_INTERFACE(struct StudentInterface, cheat_student)->print_score(cheat_student);AS_INTERFACE(struct CheatStudentInterface, cheat_student)->cheat(cheat_student);AS_INTERFACE(struct StudentInterface, cheat_student)->print_score(cheat_student);AS_INTERFACE(struct StudentInterface, cheat_student)->destroy(cheat_student);return 0; }
Listing 6: 输出
Alice 90 Bob 90 100(cheat)
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