Re: invalid covariant return type

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 07 Dec 2007 01:35:25 +0100

Message-ID:

<13lh56m7r1qblbd@corp.supernews.com>

* Sebastian Schucht:

I have a templateclass with [pure] virtual member-functions, so i can't create
instances ... but for returning the result i would like use one. So i
replaced the A<TType> from the baseclass with the Type from the spezialized
class. After this, the error invalid covariant return type occoured.

the following minimal sample shows the problem:

#include <iostream>
class C{};

template<class TType>
class A
{
  public:
  virtual A<TType> operator+(const A<TType>& i_sum) = 0;
};

class B:public A<C>
{
  public:
  B operator+(const A<C>& i_sum) {return *this;};
};

Depends what your intent is.

You could do something like

   class B: public A<C>
   {
   public:
       B( A<C> const& ) {}

       B operator+( B const& ) { return *this; }

       virtual A<C> operator+( A<C> const& x )
       { return operator+( B(x) ); }
   }

but that involves a slice for the function result.

Most probably what you really want is simply to support '+', and in that
case you don't want the virtual mechanism more than you want it for '='.

Possibly with this input you'll understand your design better so that
you can express whatever it is you want.

Cheers, & hth.,

- Alf

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