Reusing a Pointer to a Class in a Loop

Question

In a program there is a pointer to a class object WordList *TheList;. WordList has subclasses WordDataList and WordDataDLinkList, so in a case statement we interpret which subclass to use and how to print out the information in the list. From what I understand in the specs, each case is supposed to declare TheList as a pointer of that type and use that, and then reclaim the memory at the end of the case so that it can be used in the next iteration of the loop. When I try something like:

    while (true)
{
    displayMenu();
    cin>>selection;
    switch(selection)
    {
    case '1':
        TheList = new WordDataList;
        TheList->parseIntoList(inf);
        TheList->printIteratively();
        delete TheList;
        break;
    case '2':
        TheList = new WordDataList;
        TheList->parseIntoList(inf);
        TheList->printRecursively();
        delete TheList;
        break;
    case '3':
        TheList = new WordDataList;
        TheList->parseIntoList(inf);
        TheList->printPtrRecursively();
        delete TheList;
        break;
    case '6':
        cout<<"Goodbye"<<endl;
        return 0;
    default:
        cout<<"I cannot understand "<<selection<<".  Try again."<<endl;
        break;
    } // switch
} // while

Deleting the pointer makes it so after the first run through no data appears (Menu still comes up) and option 2 ends up seg faulting. I'm modifying code my professor gave, and when he had no delete call, and new WordDataList and parseIntoList before the loop it ran fine. Any suggestions?

Added:

I'm reinitializing TheList in each case because I'll be adding 4 and 5 that will use WordDataDLinkList. If it's a pointer to WordDataList from outside the case statement, how would I change it to WordDataDLinkList inside when I need to? My professor wrote WordDataList for us:

   #include <sstream>
   #include <iostream>
   #include "WordDataList.h"

   using namespace std;

     WordDataList::WordDataList()
     { numWords=0; }

     bool WordDataList::incMatch(string temp)
     { for(int i=0; i<numWords; i++) {
         if (temp==TheWords[i].getWord()) {
           TheWords[i].incCount();
           return true;
         }
       }  
       return false;
     }

     void WordDataList::parseIntoList(ifstream &inf)
     { string temp;
       while (inf >> temp) 
         if (!incMatch(temp) && numWords < 10) {
             TheWords[numWords].setWord(temp);
             TheWords[numWords++].setCount(1);
         }
     }

     // Print the data iteratively
     void WordDataList::printIteratively()
   //  void printObjectArrayIterator(WordData TheWords[], int numWords)
   {
     cout<<"--------------------------"<<endl;
     cout<<"|Object  Array  Iterative|"<<endl;
     cout<<"|Word         Occurences |"<<endl;  
     cout<<"--------------------------"<<endl;
     for(int i=0; i<numWords; i++)
       cout<<" "<<TheWords[i]<<endl;
   }


     // Print the data recursively
     void WordDataList::printRecursivelyWorker(int numWords)
     //void printObjectArrayRecursive(WordData TheWords[], int numWords)
     {if (numWords==1) {
         cout<<"--------------------------"<<endl;
         cout<<"|Object  Array  Recursive|"<<endl;
         cout<<"|Word         Occurences |"<<endl;  
         cout<<"--------------------------"<<endl;
         cout<<" "<<TheWords[numWords-1]<<endl;
         return;
       }
       printRecursivelyWorker(numWords-1);
       cout<<" "<<TheWords[numWords-1]<<endl;
     }

     // Call worker function to print the data recursively
     void WordDataList::printRecursively()
     {  printRecursivelyWorker(numWords); }


     // Print the data recursively with a pointer
     void WordDataList::printPtrRecursivelyWorker(int numWords)
     //void printObjectArrayPointerRecursive(WordData* TheWords, int numWords)
     {if (!numWords)
       { cout<<"--------------------------"<<endl;
         cout<<"|Object  Array  Pointer  |"<<endl;
         cout<<"|Word         Occurences |"<<endl;  
         cout<<"--------------------------"<<endl;
         return;
       }
       printPtrRecursivelyWorker(numWords-1);
       cout<<" "<<*(TheWords+(numWords-1))<<endl;
     }

     // Call worker function to print the data recursively
     void WordDataList::printPtrRecursively()
     { printPtrRecursivelyWorker(numWords); }

Answer 1

I think you are confused about something else. Why do you need to keep recreating the 'WordDataList' are the parse and print methods modifiying it?

If not, just create it once, and simply use the select to choose which print function to use if any.

I would also suggest putting each of the case execution statement sets into a closure, and adding some print outs or stepping through the debugger to see what is going on. Main guesses are that your 'new' is returning NULL such that you can't call its members properly OR destructor is bad.

Answer 2

Okay, so from what I can kindof tell: Options 1-3 should select a different subclass for your pointer TheList to hold. TheList is a pointer to your base class so that's good. What I think you need to do is new the appropriate subclass in each switch statement. I.E.

case '1': TheList = new WordDataDLinkList();

EDIT: if you intend to call a different version of each member function based off the classtype you are currently using, as in WordDataDLinkList::parseIntoList(inf), instead of WordDataList::parseIntoList(inf), try reading up on polymorphism