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C++ Template problem

Started by chipmunk1886 Mar 11, 2003 at 11:38 AM 10 replies 3.2k views

Original Post

Mar 11, 2003 at 11:38 AM

there's probably a quick fix for this. My compiler (VS.net) is generating the faollowing error with my templated linked list class: error C2989: 'List' : template class has already been defined as a non-template class error C2989: 'ListItr' : template class has already been defined as a non-template class I don't understand why it's puking that up. I specified that they were templated classes by writing "template -class Object- above and using "Object" for variable definitions. Those - are greater than and less than signs, html junk, The code is divided into List.h w/ list.cpp, listItr.h, and listnode.h. the last two only have one-liner function source, so i'm not making a .cpp for them. Any ideas? [edited by - chipmunk1886 on March 11, 2003 12:42:34 PM]

CpMan

Mar 11, 2003 at 11:43 AM

Can you show us some code?

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chipmunk1886

Author

chipmunk1886

Mar 11, 2003 at 2:53 PM

i thought someone might be able to see the problem based on the description i gave above. here ya go:

List Class Declaration:

  
#include "listItr.h"
#include "listnode.h"

template <class Object>
class List
{
public:
	List();
	List(const List& rhs);
	~List();

	bool IsEmpty() const;
	void makeEmpty();
	ListItr<Object> zeroth() const;
	ListItr<Object> first() const;
	void insert(const Object& x, const ListItr<Object>& p);
	ListItr<Object> find(const Object& x)const;
	ListItr<Object> findPrevious(const Object& x) const;
	void remove(const Object& x);

	void Print()const;

	const List & operator =(const List & rhs);
 
private:
	ListNode<Object> *header;

The ListNode class:

  
template <class Object>
class ListNode
{
public:
	ListNode( const Object& theElement = Object(), ListNode* n = 0)
		:element (theElement), next(n) {}

	Object element;
	ListNode* next;
	
	friend class List;//<Object>;

	friend class ListItr;//<Object>;

 
};

The List Iterator Class

  
template <class Object>
class ListItr
{
public:
	ListItr() :current(0) {}

	bool IsPastEnd() const
	{
		return current == 0;
	}
    
	void advance()
	{
		if ( !IsPastEnd() )
			current = current->next;
	}

	const Object& retrieve() const
	{
		if ( IsPastEnd() )
		{
			//throw BadIterator();

		}

		return current->element;
	}

private:
	ListNode<Object> *current;
	ListItr(ListNode<Object> *theNode) : current(theNode){};
	friend class List<Object>; //grant access to ListItr constructor

		
};

hope that helps.

Fruny

203

Fruny

Mar 11, 2003 at 3:14 PM

Minimalistic example (works on g++)

  
template<class Obj> class Bar;

template<class Obj> class Foo
{
   friend class Bar<Obj>;
};

template<class Obj> class Bar
{
};

cgoat

Mar 11, 2003 at 3:33 PM

You have to forward declare the List class as a template before declaring it as a friend class. So in listnode.h and listItr.h, put

template<class Obj> class List;

before the declaration of each class. Then you just use:

friend class List;

inside those classes.

EDIT: Put a tested solution instead.
EDIT2: Fixed formatting.

[edited by - cgoat on March 11, 2003 4:54:28 PM]

chipmunk1886

Author

chipmunk1886

Mar 12, 2003 at 9:10 AM

thanks for the help everyone. but, unfortunately my compiler is still generating errors. I''ve tried many variations of your suggestions and no dice.

i''m not so much interested in templating as messing around with data structures, so I can just use a typedef. I was hoping that they could be templated though, darn.

MadKeithV

Mod

155

MadKeithV

Mar 12, 2003 at 9:20 AM

If list is a template class, you can _NOT_ separate it into a .h and a .cpp file. All of the implementation needs to go in the .h.

Shannon Barber

Mod

368

Shannon Barber

Mar 12, 2003 at 11:07 PM

Sure you can, you just have to use magic, er I mean "export".

DerekSaw

Mar 12, 2003 at 11:46 PM

Do VS.Net really can understand export?

SiCrane

Mod

473

SiCrane

Mar 12, 2003 at 11:58 PM

No, tbe only compiler I know of that supports export is the EDG C/C++ front end. That''s only a technicality though, because as far as I know, that front end version has yet to be implemented in any of it''s reseller''s compilers. I think Comeau''s compiler is on timeline to get compliance in about three months.

MARS_999

363

MARS_999

Mar 13, 2003 at 2:02 AM

SO MSVC++ 6.0 using export will not work? I have "The Complete Reference C++" and it talks about export keyword being added to C++ along with typename. I tried it in MSVC++ 6.0 and it wants a storage class? if I remember right? Could someone post code on the correct way to use it would be even if MSVC++ doesn''t work with it. Thanks

Qw3r7yU10p!

Mar 13, 2003 at 3:50 AM

Comeau is compliant - it supports export - look, about half way down, at ''What''s new?''

It has done for a few months.

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