LinkedList 类实现不能将变量声明为抽象类型 [英] LinkedList class implementation cannot declare variable to be of Abstract Type

问题描述

在构建我的 LinkedList 类时我哪里出错了?

Where am I going wrong in constructing my LinkedList class?

我已经在您的 LinkedList 类中重新声明了来自 IList 的纯虚拟方法，但是 LinkedList 似乎被视为一个抽象类，所以编译器似乎不允许我在我的 main 函数中创建一个 LinkedList 对象:

I've re-declared the pure-virtual methods from IList in your LinkedList class, but LinkedList seems to be getting treated like an abstract class and so the compiler doesn't seem to allow me to create a LinkedList object in my main function:

ma​​in.cpp

#include <iostream>
#include <string>    
#include "LinkedList.h"    
using namespace std;

int main()
{
   int A[] {1, 2, 3, 4, 5};
   LinkedList l(A, 5);

   cout << l.getCurrentSize()<<endl;
   l.display();

   return 0;
}

LinkedList.h

#ifndef LINKED_LIST_
#define LINKED_LIST_

#include "IList.h"

class LinkedList : public IList
{
protected:
    struct Node
    {
        int data;
        struct Node* next;
    };
    struct Node* first;

public:
    // constructor
    LinkedList() { first = nullptr; }
    LinkedList(int A[], int n);

    // accessors    void display();
    virtual int getCurrentSize();

    //destructor
    virtual ~LinkedList();
};

#endif

LinkedList.cpp

#include <iostream>
#include <string>
#include "LinkedList.h"
using namespace std;

// constructor
LinkedList::LinkedList(int A[], int n)
{
    Node* last, * t;
    int i = 0;
    first = new Node;
    first->data = A[0];
    first->next = nullptr;
    last = first;
    for (i = 1; i < n; i++) 
    {
        t = new Node;
        t->data = A[i];
        t->next = nullptr;
        last->next = t;
        last = t;
    }
};

// destructor
LinkedList::~LinkedList()
{
    Node* p = first;
    while (first) {
        first = first->next;
        delete p;
        p = first;
    }
}

void LinkedList::display()
{
    Node* p = first;

    while (p) 
    {
        cout << p->data >> " ";
        p = p->next;
    }
    cout << endl;
}

int LinkedList::getCurrentSize() const
{
    Node* p = first;
    int len = 0;
    while (p) 
    {
        len++;
        p = p->next;
    }
    return len;
}

IList.h

//  Modified from code created by Frank M. Carrano and Timothy M. Henry.
//  Copyright (c) 2017 Pearson Education, Hoboken, New Jersey.

#ifndef I_LIST_
#define I_LIST_

class IList
{
public:
   /** Constructor */
   IList () : traverseCount(0) { }
    
   /** Destroys object and frees memory allocated by object.
    (See C++ Interlude 2) */
   virtual ~IList () { }

   /** Gets the current number of entries in this list.
    @return The integer number of entries currently in the list. */
   virtual int getCurrentSize() const = 0;
   
   /** Sees whether this list is empty.
    @return True if the list is empty, or false if not. */
   virtual bool isEmpty() const = 0;
   
   /** Adds a new entry to this list.
    @post  If successful, newEntry is stored in the list and
       the count of items in the list has increased by 1.
    @param newEntry  The object to be added as a new entry.
    @return  True if addition was successful, or false if not. */
   virtual bool add(int newEntry) = 0;
   
   /** Removes one occurrence of a given entry from this list,
       if possible.
    @post  If successful, anEntry has been removed from the list
       and the count of items in the list has decreased by 1.
    @param anEntry  The entry to be removed.
    @return  True if removal was successful, or false if not. */
   virtual bool remove(int anEntry) = 0;
   
   /** Removes all entries from this list.
    @post  List contains no items, and the count of items is 0. */
   virtual void clear() = 0;
   
   /** Tests whether this list contains a given entry.
    @param anEntry  The entry to locate.
    @return  True if list contains anEntry, or false otherwise. */
   virtual bool contains(int anEntry) = 0;
    
   /** Get the count of number of nodes traversed.
    @return  The integer number of nodes traversed since last time the count was reset. */
    virtual int getTraverseCount() const { return traverseCount; }
     
   /** Reset the count of nodes traversed to zero. */
    virtual void resetTraverseCount() { traverseCount = 0; }
    
protected:
    int traverseCount;
}; // end IList

#endif

推荐答案

你的 IList 是一个抽象类，有六个纯虚成员函数.为了创建派生实例(即LinkedList)，您需要也在孩子内部实现这些功能.

Your IList is an abstract class with six pure virtual member functions. In order to create the instance of the derived one (i.e. LinkedList) you need to implement those functions inside the child as well.

class LinkedList : public IList
{
    // ..... other members

public:
    // ..... other members
    virtual int getCurrentSize() const override;
    bool isEmpty() const override { // implementation  }
    bool add(int newEntry) override {// implementation }
    bool remove(int anEntry) override { // implementation }
    bool contains(int anEntry) override { // implementation }
    void clear() override { // implementation}
};

还建议使用 override 说明符覆盖基类中的虚函数，以便编译器和读者都可以轻松识别它们，而无需查看基类.

Also recommended using the override specifier to override the virtual functions from the base class, so that both the compiler and the reader can easily recognize them, without looking into to the base.

其他问题:

• 另请注意，您在子类(即 LinkedList)中的 getCurrentSize() 函数声明缺少 const.
• cout 中的错字<<数据 >>""; 应该是 cout <<p-＞数据＜＜"";
• 为什么使用命名空间标准"被认为是不好的做法?

• Also note that your getCurrentSize() functions declaration in the child class (i.e. LinkedList) lacks a const.
• Typo at cout << p -> data >> " "; should be cout << p->data << " ";
• Why is "using namespace std;" considered bad practice?

这篇关于LinkedList 类实现不能将变量声明为抽象类型的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！