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

[s.hanson]

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

#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);
	}
};

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.

[ajbharani]

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

Bharani

[exterminator]

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

[exterminator]

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

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

[s.hanson]

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

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

[Paul McKenzie]

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

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

[ajbharani]

Okie. Thanks

[exterminator]

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

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

[ajbharani]

Yes. It is