C++ 继承无法正常工作

Question

#include <iostream>
using namespace std;

class Item {
private:
    int _code;
    int _color;
    int _brand;
    double _height;
    double _length;
    double _width;
    double _weight;
    double _price;
    int _type;
    bool _doesItHaveThis;

public:
    Item();
    Item(int code, int color, int brand, double height, double length, double width,
        double weight, double price, int type, bool doesItHaveThis);
    void setCode(int code);
    void setColor(int color);
    void setBrand(int brand);
    void setHeight(double height);
    void setLength(double length);
    void setWidth(double width);
    void setWeight(double weight);
    void setPrice(double price);
    void setType(int type);
    void setDoesItHaveThis(bool doesItHaveThis);
    int getCode();
    int getColor();
    int getBrand();
    double getHeight();
    double getLength();
    double getWidth();
    double getWeight();
    double getPrice();
    int getType();
    bool getDoesItHaveThis();
    virtual ~Item();
    void display();
};
//----------------------------------------------------------
Item::Item()
{
    _code = 0;
    _color = 0;
    _brand = 0;
    _height = 0;
    _length = 0;
    _width = 0;
    _weight = 0;
    _price = 0;
    _type = 0;
    _doesItHaveThis = 0;
}
//----------------------------------------------------------
Item::Item(int code, int color, int brand, double height, double length, double width,
    double weight, double price, int type, bool doesItHaveThis)
{
    _code = code;
    _color = color;
    _brand = brand;
    _height = height;
    _length = length;
    _width = width;
    _weight = weight;
    _price = price;
    _type = type;
    _doesItHaveThis = doesItHaveThis;
}
//----------------------------------------------------------
void Item::setCode(int code)
{
    _code = code;
}
//----------------------------------------------------------
void Item::setColor(int color)
{
    _color = color;
}
//----------------------------------------------------------
void Item::setBrand(int brand)
{
    _brand = brand;
}
//----------------------------------------------------------
void Item::setHeight(double height)
{
    _height = height;
}
//----------------------------------------------------------
void Item::setLength(double length)
{
    _length = length;
}
//----------------------------------------------------------
void Item::setWidth(double width)
{
    _width = width;
}
//----------------------------------------------------------
void Item::setWeight(double weight)
{
    _weight = weight;
}
//----------------------------------------------------------
void Item::setPrice(double price)
{
    _price = price;
}
//----------------------------------------------------------
void Item::setType(int type)
{
    _type = type;
}
//----------------------------------------------------------
void Item::setDoesItHaveThis(bool doesItHaveThis)
{
    _doesItHaveThis = doesItHaveThis;
}
//----------------------------------------------------------
int Item::getCode()
{
    return _code;
}
//----------------------------------------------------------
int Item::getColor()
{
    return _color;
}
//----------------------------------------------------------
int Item::getBrand()
{
    return _brand;
}
//----------------------------------------------------------
double Item::getHeight()
{
    return _height;
}
//----------------------------------------------------------
double Item::getLength()
{
    return _length;
}
//----------------------------------------------------------
double Item::getWidth()
{
    return _width;
}
//----------------------------------------------------------
double Item::getWeight()
{
    return _weight;
}
//----------------------------------------------------------
double Item::getPrice()
{
    return _price;
}
//----------------------------------------------------------
int Item::getType()
{
    return _type;
}
//----------------------------------------------------------
bool Item::getDoesItHaveThis()
{
    return _doesItHaveThis;
}
//----------------------------------------------------------
Item::~Item()
{
    cout << "ITEM ELIMINATED" << endl;
}
//----------------------------------------------------------
void Item::display()
{
    cout << "code = " << _code << ", color = " << _color << ", brand = "
         << _brand << ", height = " << _height << ", length = " << _length
         << ", width = " << _width << ", weight = " << _weight << ", price = "
         << _price << ", type = " << _type << ", doesItHaveThis = "
         << _doesItHaveThis << endl;
}
//----------------------------------------------------------
class Pens : public Item {
private:
    int _code;
    int _color;
    int _brand;
    double _height;
    double _length;
    double _width;
    double _weight;
    double _price;
    int _type;
    bool _doesItHaveThis;
    int _packetSize;

public:
    Pens();
    Pens(int code, int color, int brand, double height, double length, double width,
        double weight, double price, int type, bool doesItHaveThis, int packetSize);
    void setPacketSize(int packetSize);
    int getPacketSize();
    virtual ~Pens();
    void display();
};
//----------------------------------------------------------
Pens::Pens()
{
    _code = 0;
    _color = 0;
    _brand = 0;
    _height = 0;
    _length = 0;
    _width = 0;
    _weight = 0;
    _price = 0;
    _type = 0;
    _doesItHaveThis = 0;
    _packetSize = 0;
}
//----------------------------------------------------------
Pens::Pens(int code, int color, int brand, double height, double length, double width,
    double weight, double price, int type, bool doesItHaveThis, int packetSize)
{
    _code = code;
    _color = color;
    _brand = brand;
    _height = height;
    _length = length;
    _width = width;
    _weight = weight;
    _price = price;
    _type = type;
    _doesItHaveThis = doesItHaveThis;
    _packetSize = packetSize;
}
//----------------------------------------------------------
void Pens::setPacketSize(int packetSize)
{
    _packetSize = packetSize;
}
//----------------------------------------------------------
int Pens::getPacketSize()
{
    return _packetSize;
}
//----------------------------------------------------------
Pens::~Pens()
{
    cout << "PEN ELIMINATED" << endl;
}
//----------------------------------------------------------
void Pens::display()
{
    cout << "code = " << _code << ", color = " << _color << ", brand = "
         << _brand << ", height = " << _height << ", length = " << _length
         << ", width = " << _width << ", weight = " << _weight << ", price = "
         << _price << ", type = " << _type << ", doesItHaveThis = "
         << _doesItHaveThis << ", packetSize = " << _packetSize << endl;
}
//----------------------------------------------------------
void main()
{
    Pens I1(1, 2, 3, 4.1, 2.0, 3.4, 3.3, 3.2, 5, 1, 0);
    I1.setBrand(999);
    I1.setDoesItHaveThis(0);
    I1.setHeight(34.62);
    I1.display();
}

这是我的代码，我想知道为什么 Pens 类没有正确地继承 Item 类的公共(public)方法。当我运行这段代码时，setBrand(999)、setDoesItHaveThis 和 setHeight 从输出中可以看出不起作用。谁能告诉我做错了什么？

Answer 1

使用多态的方法是继承基类的成员。如果您改为重复它们，那么这将声明同名的另一个成员。

struct base
{
  int member1 = 0;               // the =0 means that these members 
  int member2 = 0;               // will be default initialised to 0
  base() = default;
  explicit base(int i, int j=0)
  : member1(i), member2(j) {}
};

struct derived
: base
{
  int member1 = 1;               // not to be confused with base::member1
  int member3 = 4;
  derived() = default;
  explicit derived(int i, int j=0, int k=1, int m=4)
  : base(i,j), member1(k), member3(m) {}
  int foo() const
  { return member1 * base::member1; }
};

这里，derived有两个成员member1:一个继承自base，另一个不继承。这两者很容易被程序员混淆(但不会被编译器混淆)。所以应该避免这样的结构(好的编译器会警告你)。