Re: invalid covariant return type

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 06 Dec 2007 18:51:37 -0500

Message-ID:

<fja1v0$q0l$1@murdoch.acc.Virginia.EDU>

Sebastian Schucht wrote:

Hi,

I have a templateclass with 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;};
};

main()
{
  B a,b,c;
  a+b;
}

After compiling with g++ version 4.1.2 (Ubuntu 4.1.2-0ubuntu4) i have the
following erros on screen:

mini.cpp:14: error: invalid covariant return type for 'virtual B
B::operator+(const A<C>&)'
mini.cpp:8: error: overriding 'A<TType> A<TType>::operator+(const
A<TType>&) [with TType = C]'

- Why is this an error?

Because of clause [10.3/10]:

  The return type of an overriding function shall be either identical to the
  return type of the overridden function or covariant with the classes of
  the functions. If a function D::f overrides a function B::f, the return
  types of the functions are covariant if they satisfy the following
  criteria:
  ? both are pointers to classes or references to classes
  ...

As you can see, it only works for pointers or references.

- On wich way i can fix it?

Return a reference.

Now, that will very likely not work for operator+ (if it does something
remotely similar to adding things). However, you could do it for operator+=
because that has a reasonable claim to return *this.

Better advice might be possible if you provide some background as to what
the underlying problem is that led you to consider a virtual operator+ in
the first place.

Best

Kai-Uwe Bux