在C++中，我可以像在Java中一样拥有一个接口变量吗

Question

在Java中，我们可以在不实例化的情况下声明接口变量。这可以帮助我们处理一些抽象的概念。在下面的Java示例中，我定义了一个接口UShape和两个UShape类Rectangle和Triangle。在测试类bucket中，我能够定义一个名为myshape的私有接口变量，并使用该变量进行操作。

interface UShape {
    void setwidth(int i);
    void setheight(int i);
    int getarea();
}
class Rectangle implements UShape{
    private int w, h;
    @Override
    public void setwidth(int i) {
        this.w = i;
    }
    @Override
    public void setheight(int i) {
        this.h = i;
    }
    @Override
    public int getarea() {
        return this.w * this.h;
    }
}
class Triangle implements UShape{
    private int w, h;
    @Override
    public void setwidth(int i) {
        this.w = i;
    }
    @Override
    public void setheight(int i) {
        this.h = i;
    }
    @Override
    public int getarea() {
        return this.w * this.h / 2;
    }
}
class bucket{
    private UShape myshape;
    //do something with myshape
    public void defineShape(int i){
        if (i == 1){
            myshape = new Rectangle();
            myshape.setwidth(5);
            myshape.setheight(5);
        }
    }
    public void printArea(){
        System.out.println(myshape.getarea());
    }
}

public class TestShape {
    public static void main(String[] args){
        bucket b = new bucket();
        b.defineShape(1);
        b.printArea();
    }
}

---

我现在的问题是，在C++中，我们如何实现这个程序？我检查了一下，发现C++中的抽象类不能用来声明像UShape myshape这样的变量。我们可以使用其他方法来实现一个类似类的带有接口变量的bucket吗？

Answer 1

也许我误解了你的问题，但你没有理由不能声明抽象类类型的私有成员。以下内容应该与您的Java示例类似：

class UShape {
  public:
     virtual void setWidth (int w) =0;
     virtual void setHeight (int h) =0;
     virtual int getArea() =0;
     virtual ~UShape() =0;
};
class Rectangle: public UShape {
  private:
     int width;
     int height;
  public:
     void setWidth (int w) { this.width = w; }
     void setHeight (int h) { this.height = h; }
     int getArea (void) { return (width * height); }
};
class Triangle: public UShape {
  private:
     int width;
     int height;
  public:
     void setWidth (int w) { this.width = w; }
     void setHeight (int h) { this.height = h; }
     int getArea (void) { return (width * height) / 2; }
};
class bucket{
  private: 
    std::unique_ptr<UShape> myshape;
  public:
    //do something with myshape
    void defineShape(int i){
       if (i == 1){
          myshape = std::make_unique<Rectangle>();
          myshape->setWidth(5);
          myshape->setHeight(5);
       }
    }
    void printArea(){
        cout << myshape ? myshape->getArea() : 0;
    }
}
int main () {
   bucket b();
   b.defineShape(1);
   b.printArea();
}

Answer 2

我将使用与Mike相同的想法，但使用C++11和C++14原语进行内存管理，即在不显式调用delete运算符的情况下清理分配给指针的内存。

class UShape {
    public:
    virtual void setWidth (int w) =0;
    virtual void setHeight (int h) =0;
    virtual int getArea() =0;
};
class Rectangle: public UShape {
     private:
         int width;
         int height;
     public:
         void setWidth (int w) { this.width = w; }
         void setHeight (int h) { this.height = h; }
         int getArea (void) { return (width * height); }
};
class Triangle: public UShape {
    private:
        int width;
        int height;
    public:
        void setWidth (int w) { this.width = w; }
        void setHeight (int h) { this.height = h; }
        int getArea (void) { return (width * height) / 2; }
};
class bucket{
    private: 
        std::unique_ptr<UShape> myshape; //c++11
    public:
         //do something with myshape
        void defineShape(int i){
            if (i == 1){
                myshape = std::make_unique<Rectangle>(); //C++14 
                myshape->setWidth(5);
                myshape->setHeight(5);
            }
        }
void printArea(){
    cout << myshape->getArea();
}

}

int main () {
    std::unique_ptr<bucket> b(new bucket()); //c++11
    b->defineShape(1);
    b->printArea();
}

Answer 3

在C++中，您必须使用指向基类的指针，或者更好的是指向智能指针，如shared_ptr或unique_ptr:

class bucket{
private:
    std::unique_ptr<UShape> myshape;
public:
    //do something with myshape
    void defineShape(int i){
        if (i == 1){
            myshape = std::make_unique<Rectangle>();
            myshape->setwidth(5);
            myshape->setHeight(5);
        }
    }
    void printArea(){
        std::cout << myshape->getarea() << std::endl;
    }
};
int main() {
    bucket b;
    b.defineShape(1);
    b.printArea();
}

在某些情况下，参考资料也很有用：

Rectangle r;
UShape& shape = r;
std::cout << shape.getarea() << std::endl;