Re: invalid covariant return type
Sebastian Schucht wrote:
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.
"Covariant" is either a pointer or a reference. With objects, they
have to be _exactly the same_ to be covariant.
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;
What's the 'c' object for?
}
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?
A<C> is not covariant with B, they are not the same type. A<C>& would
be covariant with B&. A<C>* would be covariant with B*.
- On wich way i can fix it?
You can't. Don't make operators virtual, it's unnatural since they
return objects (where slicing will most likely occur).
V
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