Bag ADT C++ 的链接数据结构实现问题，程序中止

如何解决Bag ADT C++ 的链接数据结构实现问题，程序中止

所以我尝试使用带有单元格数组的链接数据结构在 C++ 中实现 Bag ADT。我已经编辑了 ArrayLinkedBag.cpp 和头文件，但是我一直遇到奇怪的错误，我想知道是否有人可以帮助我找出我出错的地方。当我尝试进行测试时，由于某种原因整个过程中止，我无法查明错误。我已经尝试了一切来查明问题，包括重写所有内容并逐行测试行，但它只是不起作用。任何帮助将不胜感激。

ArrayLinkedBag.cpp

/** Implementation file for the class ArrayLinkedBag.
  @file ArrayLinkedBag.cpp */

#include "ArrayLinkedBag.h"
#include <cstddef>

template<class ItemType>
ArrayLinkedBag<ItemType>::ArrayLinkedBag(): head(-1),free(0),itemCount(0)
{
    //create free list
    for (int i = 0; i < DEFAULT_CAPACITY; i++)
    {
        cells[i] = (Cell<ItemType>){"a",i+1};
    }

}  // end default constructor

template<class ItemType>
int ArrayLinkedBag<ItemType>::getCurrentSize() const
{
    return itemCount;
}  // end getCurrentSize

template<class ItemType>
bool ArrayLinkedBag<ItemType>::isEmpty() const
{
    return itemCount == 0;
}  // end isEmpty

template<class ItemType>
bool ArrayLinkedBag<ItemType>::add(const ItemType& newEntry)
{
    bool hasRoomToAdd = (itemCount < DEFAULT_CAPACITY);
    if (hasRoomToAdd)
    {
        int newFree=cells[free].next;
        cells[free]=(Cell<ItemType>){newEntry,head};
        head=free;
        itemCount++;
        free=newFree;
    }  // end if

    return hasRoomToAdd;
}  // end add

template<class ItemType>
bool ArrayLinkedBag<ItemType>::remove(const ItemType& anEntry)
{
    int locatedindex = getIndexOf(anEntry);
    bool canRemoveItem = !isEmpty() && (locatedindex > -1);
    if (canRemoveItem)
    {
        itemCount--;
        cells[locatedindex] = (Cell<ItemType>){NULL,free};
        free=locatedindex;
    }  // end if

    return canRemoveItem;
}  // end remove

template<class ItemType>
void ArrayLinkedBag<ItemType>::clear()
{
    //reset free list 
    for (int i = 0; i < DEFAULT_CAPACITY; i++)
    {
        cells[i] = (Cell<ItemType>){"a",i+1};
    }
    head=-1;
    free=0;
    itemCount = 0;
}  // end clear

template<class ItemType>
bool ArrayLinkedBag<ItemType>::contains(const ItemType& anEntry) const
{
    return getIndexOf(anEntry) > -1;
}  // end contains

template<class ItemType>
int ArrayLinkedBag<ItemType>::getFrequencyOf(const ItemType& anEntry) const
{
    int frequency = 0;
    int curIndex = 0;       // Current array index
    while (curIndex < itemCount)
    {
        if (cells[curIndex].item == anEntry)
        {
            frequency++;
        }  // end if

        curIndex++;          // Increment to next entry
    }  // end while

    return frequency;
}  // end getFrequencyOf

template<class ItemType>
std::vector<ItemType> ArrayLinkedBag<ItemType>::toVector() const
{
    std::vector<ItemType> bagContents;
    for (int i = 0; i < itemCount; i++)
        bagContents.push_back(cells[i].item);

    return bagContents;
}  // end toVector

// private
template<class ItemType>
int ArrayLinkedBag<ItemType>::getIndexOf(const ItemType& target) const
{
    bool found = false;
    int result = -1;
    int searchIndex = 0;

    // If the bag is empty,itemCount is zero,so loop is skipped
    while (!found && (searchIndex < itemCount))
    {
        if (cells[searchIndex].item == target)
        {
            found = true;
            result = searchIndex;
        }
        else
        {
            searchIndex++;
        }  // end if
    }  // end while

    return result;
}  // end getIndexOf

ArrayLinkedBag.h

/** Header file for an array-based linked list implementation of the ADT bag.
  @file ArrayLinkedBag.h */

#ifndef ARRAY_LINKED_BAG_
#define ARRAY_LINKED_BAG_

#include "BagInterface.h"

template<class ItemType>
class ArrayLinkedBag : public BagInterface<ItemType>
{
private:
    static const int DEFAULT_CAPACITY = 6;

    template<class T>
    struct Cell {
        T item;
        int next;
    };

    Cell<ItemType> cells[DEFAULT_CAPACITY];
    int head;
    int itemCount;                         // Current count of bag items 
    int free;
    // Returns either the index of the element in the array items that
    // contains the given target or -1,if the array does not contain
    // the target.
    int getIndexOf(const ItemType& target) const;
public:
    ArrayLinkedBag();
    int getCurrentSize() const;
    bool isEmpty() const;
    bool add(const ItemType& newEntry);
    bool remove(const ItemType& anEntry);
    void clear();
    bool contains(const ItemType& anEntry) const;
    int getFrequencyOf(const ItemType& anEntry) const;
    std::vector<ItemType> toVector() const;


};

#include "ArrayLinkedBag.cpp"
#endif

错误信息

g++ -Wall -Wextra -g src/test_driver.cpp -o build/test_driver
In file included from src/test_driver.cpp:7:
src/ArrayLinkedBag.cpp: In instantiation of ‘ArrayLinkedBag<ItemType>::ArrayLinkedBag() [with ItemType = std::__cxx11::basic_string<char>]’:
src/test_driver.cpp:91:48:   required from here
src/ArrayLinkedBag.h:24:9: warning: ‘ArrayLinkedBag<std::__cxx11::basic_string<char> >::free’ will be initialized after [-Wreorder]
     int free;
         ^~~~~
src/ArrayLinkedBag.h:22:9: warning:   ‘int ArrayLinkedBag<std::__cxx11::basic_string<char> >::head’ [-Wreorder]
     int head;
         ^~~~
In file included from src/ArrayLinkedBag.h:43,from src/test_driver.cpp:7:
src/ArrayLinkedBag.cpp:8:1: warning:   when initialized here [-Wreorder]
 ArrayLinkedBag<ItemType>::ArrayLinkedBag():free(0),head(0),itemCount(0){
 ^~~~~~~~~~~~~~~~~~~~~~~~
[username testassignment3]$ make test
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
/bin/sh: line 1: 98661 Aborted                 (core dumped) ./build/test_driver ARRAY_LINKED > output/array_linked.out
make: *** [test] Error 134