无法分配抽象类型 SortedListHasA 的对象

Question

我在网上做了一些研究，但仍然无法解决我在作业中遇到的问题。

编译器给出的错误是：

test.cpp: In function ‘int main()’:
test.cpp:13:39: error: cannot allocate an object of abstract type ‘SortedListHasA<std::basic_string<char> >’
  testPtr = new SortedListHasA<string>();
                                       ^
In file included from test.cpp:2:0:
SortedListHasA.h:10:7: note:   because the following virtual functions are pure within ‘SortedListHasA<std::basic_string<char> >’:
 class SortedListHasA : public SortedListInterface<ItemType>
       ^
In file included from SortedListHasA.h:5:0,
                 from test.cpp:2:
SortedListInterface.h:35:13: note:  int SortedListInterface<ItemType>::getPosition(const ItemType&) [with ItemType = std::basic_string<char>]
 virtual int getPosition( const ItemType& anEntry) = 0;
             ^

我认为编译器在抱怨如何在派生类中没有重新定义虚函数，但我在 LinkList.cpp 文件中做了，代码所在的位置：

template < class ItemType> 
int LinkedList<ItemType>::getPosition(const ItemType& anEntry) const
{ 
    int position = 0; 
    if (!isEmpty()) 
    { 
        int index = 0; 
        Node<ItemType>* curPtr = headPtr; 
        while ((position == 0) && (index < itemCount)) // While not found 
        { 
            if (anEntry == curPtr->getItem()) 
            position = index + 1; // anEntry is located 
            else 
            { 
                index++; 
                curPtr = curPtr->getNext(); 
            } // end if 
        } // end while 
    } // end if 

 return position; 
}

我的测试代码如下所示：

int main()
{

    SortedListInterface<string>* testPtr = NULL;

    testPtr = new SortedListHasA<string>();
/*
    testPtr -> insertSorted("add");
    testPtr -> insertSorted("subtrac");
*/
    return 0;
}

类 SortedListHasA 是：

/** ADT sorted list using the ADT list.
@file SortedListHasA.h */
#ifndef _SORTED_LIST_HAS_A
#define _SORTED_LIST_HAS_A
#include "SortedListInterface.h"
#include "ListInterface.h"
#include "Node.h"
#include "PrecondViolatedExcep.h"
template < class ItemType>
class SortedListHasA : public SortedListInterface<ItemType>
{
private :
    ListInterface<ItemType>* listPtr;
public :
    SortedListHasA();
    SortedListHasA(const SortedListHasA<ItemType>& sList);
    virtual ~SortedListHasA();
    void insertSorted( const ItemType& newEntry);
    bool removeSorted( const ItemType& anEntry);
    int getPosition( const ItemType& newEntry) const;
    // The following methods have the same specifications
    // as given in ListInterface:
    bool isEmpty() const ;
    int getLength() const ;
    bool remove(int position);
    void clear();
    ItemType getEntry( int position) const throw(PrecondViolatedExcep);
}; // end SortedListHasA
#include "SortedListHasA.cpp"
#endif

派生自SortedListInterface.h：

/** Interface for the ADT sorted list
@file SortedListInterface.h */
#ifndef _SORTED_LIST_INTERFACE
#define SORTED_LIST_INTERFACE
template < class ItemType>
class SortedListInterface
{
public :

virtual bool removeSorted( const ItemType& anEntry) = 0;

virtual int getPosition( const ItemType& anEntry) = 0;

virtual bool isEmpty() const = 0;

virtual int getLength() const = 0;

virtual bool remove( int position) = 0;
virtual void clear() = 0;

virtual ItemType getEntry( int position) const = 0;
}; // end SortedListInterface
#endif

感谢任何帮助和建议！

Answer 1

OK 事情已解决，只需更改为：

virtual int getPosition( const ItemType& anEntry) const = 0;

在class SortedListInterface 定义中。您在成员函数 getPosition 的声明中缺少 const 限定符。