Re: covariant template problem

From:
Kai-Uwe Bux <jkherciueh@gmx.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 28 Feb 2008 06:20:09 -0500
Message-ID:
<fq65da$gt4$1@aioe.org>
weinma81@gmail.com wrote:

Hi!

I'm writing a vector-class for some calculations and it look's like i
am having a covariant problem, where i shouldn't:

The Vector-Class Look's like this and works:

template <int _size,class T>
class Vector {
protected:
T vector[_size];
public:
     ...
     virtual Vector<_size,T> operator-(const Vector<_size,T> &v)
const; // that's one of the methods causing problems


Why is that method virtual? Virtual binary operators do not really work all
that well.

As the crossproduct is only defined for a 3D Vector i wanted to write
and inherit a Vector3 from Vector.


Don't. Just have a free standing function:

  template < typename T >
  Vector<3,T>
  cross_product ( Vector<3,T> const & lhs, Vector<3,T> const & rhs ) {
    ...
  }

Note that with the inheritance approach, you create two types: Vector3<T>
and Vector<3,T>. These types will not interact all that nicely.

The operators should also return the Vector3 so i overwrote them.. and
there comes my covariant problem.
Looks like this:

template <class T>
class Vector3 : public Vector<3,T> {
public:
 ...
 Vector3<T> operator-(const Vector<3,T> &v) const; // <-- Theres my
problem

Compiler cries:
../Vector3.h:56: error: invalid covariant return type for 'Vector3<T>
Vector3<T>::operator-(const Vector<3, T>&) const [with T = float]'
../Vector.h:72: error: overriding 'Vector<_size, T> Vector<_size,
T>::operator-(const Vector<_size, T>&) const [with int _size = 3, T =
float]'

don't know what to do.. asked some people but they also said that this
should work..


Covariant return types are restricted to pointers and references to classes.
See [10.3/5] for details.

Best

Kai-Uwe Bux

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