C++ 无法覆盖纯虚函数

Question

SortedList.h是抽象模板(纯虚函数)，LinkedSortedList.h派生SortedList.h是模板，LinkedSortedList.cpp是实现LinkedSortedList.h中函数的模板。

我的错误是当我试图在 LinkedSortedList.h 中覆盖 SortedList.h 中的函数时。

SortedList.h 中的方法:

virtual void clear() = 0;   

LinkedSortedList.h中的方法:

template <typename Elm> void SortedList<Elm>::clear() override; 

错误:

error C3240: 'clear' : must be a non-overloaded abstract member function of 'SortedList'

error C2838: 'clear' : illegal qualified name in member declaration

我试过:

void SortedList<Elm>::clear(){}

但我仍然得到同样的错误。我试图在网上找到解决方案，但失败了。

这里是SortedList.h，一定不能改:

#ifndef _SortedListClass_
#define _SortedListClass_

template <class Elm> class SortedList {
public:

  // -------------------------------------------------------------------
  // Pure virtual functions -- you must implement each of the following
  // functions in your implementation:
  // -------------------------------------------------------------------

  // Clear the list.  Free any dynamic storage.
  virtual void clear() = 0;          
                                 
  // Insert a value into the list.  Return true if successful, false
  // if failure.
  virtual bool insert(Elm newvalue) = 0;
            
  // Get AND DELETE the first element of the list, placing it into the
  // return variable "value".  If the list is empty, return false, otherwise
  // return true.
  virtual bool getfirst(Elm &returnvalue) = 0;

  // Print out the entire list to cout.  Print an appropriate message
  // if the list is empty.  Note:  the "const" keyword indicates that
  // this function cannot change the contents of the list.
  virtual void print() const = 0;

  // Check to see if "value" is in the list.  If it is found in the list,
  // return true, otherwise return false.  Like print(), this function is
  // declared with the "const" keyword, and so cannot change the contents
  // of the list.
  virtual bool find(Elm searchvalue) const = 0;

  // Return the number of items in the list
  virtual int size() const = 0;
};

#endif

这里是 LinkedSortedList.h 如果需要可以重写:

#ifndef _LinkedSortedList_
#define _LinkedSortedList_

#include "SortedList.h"
#include "LinkedNode.h"
#include <iostream>

using namespace std;

template <class Elm> class LinkedSortedList:public SortedList<Elm>
{
public:
    template <typename Elm> LinkedSortedList();
    virtual void SortedList<Elm>::clear();          
    virtual bool SortedList<Elm>::insert(Elm newvalue);
    virtual bool SortedList<Elm>::getfirst(Elm &returnvalue);
    virtual void SortedList<Elm>::print() const;
    virtual bool SortedList<Elm>::find(Elm searchvalue) const;
    virtual int SortedList<Elm>::size() const;

private:
    LinkedNode<Elm> *head;
    int num;
};

#endif

LinkedSortedList.cpp 是 LinkedSortedList 的实现，最后有这两行:

template class LinkedSortedList<int>;
template class LinkedSortedList<double>;

Answer 1

您的覆盖不正确。覆盖是派生类的成员，而不是基类。

此外，您的构造函数没有理由成为函数模板。

我认为你的类应该看起来更像这样:

template <class Elm> class LinkedSortedList : public SortedList<Elm>
{
public:
    LinkedSortedList();
    void clear() override;          
    bool insert(Elm newvalue) override;
    bool getfirst(Elm &returnvalue) override;
    void print() const override;
    bool find(Elm searchvalue) const override;
    int size() const override;

private:
    LinkedNode<Elm> *head;
    int num;
};