C语言中的面向对象？从头开始实现接口

本文探讨如何在C语言中模拟面向对象编程中的接口概念。我们将以计算车辆价格为例，分别用Java和C语言实现，对比两种语言的差异，并展示如何在C中实现接口的基本功能。

Java实现：

Java中，接口使用interface关键字定义，类通过implements关键字实现接口。示例代码如下：

interface Vehicle {
    int price();
}

class Car implements Vehicle {
    private final int speed;

    public Car(int speed) {
        this.speed = speed;
    }

    @Override
    public int price() {
        return speed * 60;
    }
}

class Motorcycle implements Vehicle {
    private final int cc;

    public Motorcycle(int cc) {
        this.cc = cc;
    }

    @Override
    public int price() {
        return cc * 10;
    }
}

public class Main {
    public static void printVehiclePrice(Vehicle vehicle) {
        System.out.println("$"   vehicle.price()   ".00");
    }

    public static void main(String[] args) {
        Car car = new Car(120);
        Motorcycle motorcycle = new Motorcycle(1000);

        printVehiclePrice(car);
        printVehiclePrice(motorcycle);
    }
}

C语言实现：

C语言没有直接的接口机制。我们可以通过枚举类型、结构体和函数指针来模拟接口的行为。

首先定义枚举类型表示车辆类型：

typedef enum { VEHICLE_CAR, VEHICLE_MOTORCYCLE } VehicleType;

然后定义车辆结构体，包含类型信息和函数指针：

typedef struct {
    VehicleType type;
    int (*price)(void*); // 函数指针，指向价格计算函数
} Vehicle;

接下来，实现汽车和摩托车结构体：

typedef struct {
    VehicleType type;
    int speed;
} Car;

typedef struct {
    VehicleType type;
    int cc;
} Motorcycle;

相应的初始化和价格计算函数：

Car* car_init(int speed) {
    Car* car = malloc(sizeof(Car));
    car->type = VEHICLE_CAR;
    car->speed = speed;
    return car;
}

int car_price(void* car) {
    return ((Car*)car)->speed * 60;
}

Motorcycle* motorcycle_init(int cc) {
    Motorcycle* motorcycle = malloc(sizeof(Motorcycle));
    motorcycle->type = VEHICLE_MOTORCYCLE;
    motorcycle->cc = cc;
    return motorcycle;
}

int motorcycle_price(void* motorcycle) {
    return ((Motorcycle*)motorcycle)->cc * 10;
}

最后，实现vehicle_price函数，根据车辆类型调用不同的价格计算函数：

int vehicle_price(Vehicle* vehicle) {
    switch (vehicle->type) {
        case VEHICLE_CAR:
            return car_price((Car*)vehicle);
        case VEHICLE_MOTORCYCLE:
            return motorcycle_price((Motorcycle*)vehicle);
        default:
            return 0;
    }
}

void print_vehicle_price(Vehicle* vehicle) {
    printf("$%d.00\n", vehicle_price(vehicle));
}

int main() {
    Vehicle car_v = {VEHICLE_CAR, car_price};
    ((Car*)&car_v)->speed = 120; // 强制类型转换

    Vehicle motorcycle_v = {VEHICLE_MOTORCYCLE, motorcycle_price};
    ((Motorcycle*)&motorcycle_v)->cc = 1000; // 强制类型转换


    print_vehicle_price(&car_v);
    print_vehicle_price(&motorcycle_v);

    free((Car*)&car_v);
    free((Motorcycle*)&motorcycle_v);
    return 0;
}

这种C语言的实现模拟了接口的行为，但需要手动管理内存和类型转换，相比Java的接口机制更加复杂。 这种方法在处理复杂数据结构，例如抽象语法树（AST）时，可以提高代码的可维护性和可读性。

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