Hy everyone !

Here is a peace of code that leads to the following error when compiling : invalid covariant return type for ‘virtual List* Elem::list()’

[source]class AbstractList;

class AbstractElem
{
public :
virtual AbstractList* list() = 0;
protected :
AbstractElem() {};
};

class List;

class Elem : public AbstractElem
{
public :
Elem() :
AbstractElem()
{};
virtual List* list()
{
return _list;
}
protected :
List* _list;
};

class AbstractList
{
public :
virtual AbstractElem* elem() = 0;
protected :
AbstractList() {};
};

class List : public AbstractList
{
public :
List() :
AbstractList()
{}

virtual Elem* elem()
{
return _e;
}
protected :
Elem* _e;
};

int main()
{
return 0;
}[/source]

The thing that I don't understand is why I only get this error for the list() function but not for the elem() function !

Could anyone explain me the difference ?

Thanks.

Thanks for the answer ! I already tried this, separating into Elem.h and List.h and here is what I get as an error :

In file included from List.h:4,
from covariant.cpp:1:
Elem.h:9: error: ISO C++ forbids declaration of ‘AbstractList’ with no type
Elem.h:9: error: ‘AbstractList’ declared as a ‘virtual’ field
Elem.h:9: error: expected ‘;’ before ‘*’ token
Elem.h:22: error: ISO C++ forbids declaration of ‘List’ with no type
Elem.h:22: error: ‘List’ declared as a ‘virtual’ field
Elem.h:22: error: expected ‘;’ before ‘*’ token
Elem.h:27: error: expected `;' before ‘protected’
Elem.h:28: error: ISO C++ forbids declaration of ‘List’ with no type
Elem.h:28: error: expected ‘;’ before ‘*’ token

Using class Elem; and class List; instead of #include in the header file leads to the same error than before...

Any idea ?

[source lang="cpp"]#ifndef _ELEM_
#define _ELEM_

#include "List.h"

class AbstractElem
{
public :
virtual AbstractList* list() = 0;

protected :
AbstractElem() {};
};

class Elem : public AbstractElem
{
public :
Elem() :
AbstractElem()
{};

virtual List* list()
{
return _list;
}

protected :
List* _list;
};

#endif[/source]

[source lang="cpp" ]#ifndef _LIST_
#define _LIST_

#include "Elem.h"

class AbstractList
{
public :
virtual AbstractElem* elem() = 0;

protected :
AbstractList() {};
};

class List : public AbstractList
{
public :
List() :
AbstractList()
{}

virtual Elem* elem()
{
return _e;
}

protected :
Elem* _e;
};


#endif[/source]

That was just an example. I'm not trying to develop a new container, the names are arbitrary, I just though it would be easier than using A, B or C as a class name ;)
I've got this error in a much more complex example and tried to find a more simple one.

I just want to understand why I had a compilation error in re-implemented list() but not for elem() !

About the only way I can see you solving it would be to replace the Elem* 's in List with AbstractElem* 's in the List definition and List* 's with AbstractList* 's in Elem's definition.

I think I'm gonna try this way. Thanks !