继承类的指针问题

pointer problems with inherited class

本文关键字：问题 指针 继承      更新时间：2023-10-16

在下面的代码中，SecClass从FirstClass继承了'item'。然而，当我在main中运行"ClassC"时，第一项和第二项都是210。如果我从继承的类（FirstClass）中删除指针，它就会正常运行。我在指针和继承方面仍然很不稳定，所以我在这里缺少了一些东西。。。

// declaration of FirstClass
// FirstClass.h
#include <iostream>
#ifndef FIRSTCLASS_H
#define FIRSTCLASS_H
using namespace std;
class FirstClass
{
 private:
   int *item;
 public:
   FirstClass();
   FirstClass(int thatItem);
   ~FirstClass();
   int getItem();
   void setItem(int thatItem);
};
#endif

// the implementation of FirstClass.h
// FirstClass.cpp
#include "FirstClass.h"
using namespace std;
FirstClass::FirstClass()
{
int *setInitial = new int;
*setInitial = 5;
item = setInitial;
}
FirstClass::FirstClass(int thatItem)
{
item = &thatItem;
}
FirstClass::~FirstClass(){}
int FirstClass::getItem()
{
    return *item;
}
void FirstClass::setItem(int thatItem) 
{
    item = &thatItem;
}

// declaration of SecClass
// SecClass.h
#ifndef SECCLASS_H
#define SECCLASS_H
#include "FirstClass.h"
using namespace std;
class SecClass : public FirstClass
{
 private:
   int *secItem;
 public:
   SecClass();
   SecClass(int newItem, int thatItem);
   ~SecClass();
   int getSecItem();
   void setSecItem(int newItem);
};
#endif

// the implementation of SecClass.h
// SecClass.cpp
#include "SecClass.h"
using namespace std;
SecClass::SecClass()
{
int *setSecInitial = new int;
*setSecInitial = 16;
secItem = setSecInitial;
}
SecClass::SecClass(int newItem, int thatItem) : FirstClass(thatItem)
{
    secItem = &newItem;
}
SecClass::~SecClass(){}
int SecClass::getSecItem()
{
    return *secItem;
}
void SecClass::setSecItem(int newItem) 
{
    secItem = &newItem;
}

// main program
#include <iostream>
#include "FirstClass.h"
#include "SecClass.h"
using namespace std;
int main()
{
FirstClass classA;
cout << "classA item: " << classA.getItem() << endl << endl;
FirstClass classZ(86);
cout << "classZ item: " << classZ.getItem() << endl << endl;
SecClass classB;
cout << "classB first item: " << classB.getItem() << endl;
cout << "classB second item: " << classB.getSecItem() << endl << endl;
SecClass classC(72, 210);
cout << "classC first item: " << classC.getItem() << endl;
cout << "classC second item: " << classC.getSecItem() << endl;

return 0;
}

您的构造函数

void FirstClass::setItem(int thatItem) 
{
    item = &thatItem;
}

正在做一件非常糟糕的事情，即存储临时对象的地址（传递给构造函数的int）。

int将在从构造函数返回后立即被销毁，并且您的item指针将指向一个已被其他对象重用的内存区域。

当你做这种事情时，任何事情都可能发生（包括从你鼻孔里飞出来的守护进程），所以不要这样做。

这是使用指针的修改后的代码。它有效。正如你所提到的，它只是了解指针等

    class FirstClass
{
 private:
   int *item;
 public:
   FirstClass();
   FirstClass(int *thatItem);
   ~FirstClass();
   int getItem();
};
FirstClass::FirstClass()
{
item = new int(5);
}
FirstClass::FirstClass(int *thatItem)
{
item = thatItem;
}
FirstClass::~FirstClass(){}
int FirstClass::getItem()
{
    return *item;
}
class SecClass : public FirstClass
{
    private:
    int *secItem;
    public:
    SecClass();
    SecClass(int *newItem, int *thatItem);
    ~SecClass();
    int getSecItem();
};
SecClass::SecClass()
{
secItem = new int(16);
}
SecClass::SecClass(int *newItem, int *thatItem) : FirstClass(thatItem)
{
    secItem = newItem;
}
SecClass::~SecClass(){}
int SecClass::getSecItem()
{
    return *secItem;
}
int main()
{
FirstClass classA;
cout << "classA item: " << classA.getItem() << endl << endl;
int *i = new int(86);
FirstClass classZ(i);
cout << "classZ item: " << classZ.getItem() << endl << endl;
SecClass classB;
cout << "classB first item: " << classB.getItem() << endl;
cout << "classB second item: " << classB.getSecItem() << endl << endl;
int *j = new int(72);
int *k = new int(210);
SecClass classC(j,k);
cout << "classC first item: " << classC.getItem() << endl;
cout << "classC second item: " << classC.getSecItem() << endl;

return 0;
}
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