[RESOLVED] Inheritance with Templates: "Does not name a type

This is a snippet from a hash table program I am writing. It uses inheritance, where HashTable is abstract base, OpenAddressing is abstract and LinearProbeHash inherits from OpenAddressing

Code:

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#include <vector>
#include <string>
using namespace std;

enum Status { EMPTY, USED, FREE };

///////////////////////////////////////////////////////////////
//  HASH TABLE CLASS - ABSTRACT BASE              //
///////////////////////////////////////////////////////////////
template <typename Object>
class HashTable
{
public:
        struct HashElement// Storage "Bucket" for a hash element
        {
                int key;
                Object el;
                Status status;                                       
                HashElement(const int& k=0, const Object& e=NULL, Status st=EMPTY)
                : key(k),el(e), status(st) {}
        };       
        typedef HashElement* ElemPtr;

protected:
        virtual ElemPtr finder(const int& k) = 0;
        virtual ElemPtr inserter(const int& k, const Object& e) = 0;
        virtual int                hash1(const int& k) { return (k % N); }
        int sz, N;
public:
        virtual void        removeElement(int k) = 0;       
        virtual ElemPtr find(int k) { return finder(k); }

};
///////////////////////////////////////////////////////////////
//OPEN-ADDRESSING CLASS - ABSTRACT BASE      //
///////////////////////////////////////////////////////////////
template <typename Object>
class OpenAddressing : public HashTable<Object>
{
public:       
        void removeElement(int k)
        {
                ElemPtr temp = finder(k);
                *temp = this->HashElement(FREE);
                this->sz--;       
        }
protected:
        ElemPtr HashArray;
        virtual int        hash2(const int& k, const int& i) = 0;        // hash function
        ElemPtr inserter(const int& k, const Object& e)
        {
                int i = 0;
                int insertLoc = this->hash1(k);                //starts insert at hash value of key
                int startPoint = insertLoc;                //keeps track of initial insert location
                while (HashArray[insertLoc].status == USED)
                {
                        this->collisions++;                                                                //keep track of collisions for testing
                        i++;
                        insertLoc = hash2(k,i);
                        if ( insertLoc == startPoint) //loop has gone all the way around the table
                        {       
                                cerr << "Table Full!\n";
                                return NULL;
                        }
                }
                HashArray[insertLoc] = HashElement(k,e,USED);
                this->sz++;
                return &HashArray[insertLoc];
        }
        ElemPtr finder(const int& k)
        {
                int loc = this->hash1(k);
                int startPoint = loc;
                bool found = true;
                int i=0;
                while (HashArray[loc].key != k)
                {
                                i++;
                                this->collisions++;
                                loc = hash2(k,i);
                                if (HashArray[loc].status == EMPTY || startPoint == loc)
                                {
                                        found = false;
                                        break;
                                }
                }       
                if (found == false)
                {
                        cerr << "Couldn't find element with key: " << k << endl;
                        return NULL;
                }
                else
                //        cout << "\nIterations to find " << k << "\t << i << endl;
                return &HashArray[loc];               
        }
};
///////////////////////////////////////////////////////////////
//                  LINEAR PROBING CLASS - ABSTRACT BASE              //
///////////////////////////////////////////////////////////////
template <typename Object>
class LinearProbeHash : public OpenAddressing<Object>
{
public:
        LinearProbeHash(int capacity=107)
        {
                this->sz = 0;
                this->N = capacity;
                this->collisions = 0;
                this->HashArray = new HashElement[this->N];
        }
protected:
        virtual int  hash2(const int& k, const int& i)
        {
                return ((this->hash1(k)+i) % this->N);
        }
};

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I cannot figure this out. I get this error at several places:
Hash.h:55: error: ‘ElemPtr’ does not name a type
Hash.h:55: note: (perhaps ‘typename HashTable<Object>::ElemPtr’ was intended)

Does anyone know what I might be doing wrong. I have never done any serious implementation of templates with inheritance.

I tried compiling this code with VC++ 6.0
It compiles successfully.

Bharani

Do as the compiler says.. you need the 'typename' keyword with ElemPtr as noted in the error.

Quote:

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Originally Posted by ajbharani

I tried compiling this code with VC++ 6.0
It compiles successfully.

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Aah. Still with VC++ 6.0? Look out for the express editions for 2005/2008 versions. They are freely downloadable.

Quote:

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Originally Posted by exterminator

Do as the compiler says.. you need the 'typename' keyword with ElemPtr as noted in the error.

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Thanks. I don't know why I hadn't tried that.

Quote:

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Originally Posted by ajbharani

I tried compiling this code with VC++ 6.0
It compiles successfully.

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VC++ 6.0 is not correct, since it is not a conforming ANSI C++ compiler. The code that was posted is wrong.

Never use Visual C++ 6.0 to determine if templated code is correct or not.

Regards,

Paul McKenzie

Okie. Thanks :)

Quote:

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Originally Posted by ajbharani

Hope is a good thing; may be the best of all; and no good thing ever dies.

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Isn't that from The Shawshank Redemption... I love that movie. :)

Yes. It is :)