Covariants

[Noah Roberts]

I got a question about covariants and overloading. This is my code:

#include <iostream>
using namespace std;

class Parent1
{
};

class Parent2
{
public:
virtual Parent1 * f() { return new Parent1(); }
};

class Child1 : public Parent1
{
public:
class Child2 : public Parent2
{
public:
Child1 * f() { return new Child1(); }
};
};

int main()
{
int x;
cin >> x;
}

This code refuses to compile saying that overload of f() returns
non-covariant of Parent2::f()'s return.

This change compiles fine:

class Child1 : public Parent1
{
public:
};
class Child2 : public Parent2
{
public:
Child1 * f() { return new Child1(); }
};

As does this:


template<class T>
class Child1 : public Parent1
{
public:
class Child2 : public Parent2
{
public:
Child1 * f() { return new Child1(); }
};
};

int main()
{
Child1<int> c;
int x;
cin >> x;
}


I thought that any inner class had access to all names the outer class
does. The outer class knows what it is, why doesn't the inner class
know what the outer class is? Obviously Parent1 is not fully defined
when declaring Child2::f() but what is the rule I'm breaking in the
first example?

 

[Victor Bazarov]

I got a question about covariants and overloading. This is my code:

#include <iostream>
using namespace std;

class Parent1
{
};

class Parent2
{
public:
virtual Parent1 * f() { return new Parent1(); }
};

class Child1 : public Parent1
{
public:
class Child2 : public Parent2
{
public:
Child1 * f() { return new Child1(); }
};
};

int main()
{
int x;
cin >> x;
}

This code refuses to compile saying that overload of f() returns
non-covariant of Parent2::f()'s return.

Really? Comeau compiles it fine, so does VC++ v8.

This change compiles fine:

[..]

I thought that any inner class had access to all names the outer class
does. The outer class knows what it is, why doesn't the inner class
know what the outer class is? Obviously Parent1 is not fully defined
when declaring Child2::f() but what is the rule I'm breaking in the
first example?

The compiler you're using is obviously buggy.

V

 

[Noah Roberts]

Really? Comeau compiles it fine, so does VC++ v8.

The compiler you're using is obviously buggy.

Hmmm....guess it must have been fixed later...this is v7.

 

[Victor Bazarov]

Hmmm....guess it must have been fixed later...this is v7.

Try (upgrade to) v7.1 or/and ask in 'microsoft.public.vc.language'.

V

 

[Noah Roberts]

Try (upgrade to) v7.1 or/and ask in 'microsoft.public.vc.language'.

No need Vic, that wasn't a question...hense the period.

 

[Victor Bazarov]

No need Vic, that wasn't a question...hense the period.

That wasn't an answer. It was advice.

 

[Noah Roberts]

That wasn't an answer. It was advice.

For what purpose. I don't need to be told to go post elsewhere. My
question was on-topic.

 

[Victor Bazarov]

For what purpose. I don't need to be told to go post elsewhere. My
question was on-topic.

I am sorry, I didn't mean to suggest you were off-topic. I just pointed
you to that newsgroup because they know better about the differences between
v7 and v7.1 (if you don't want or can't upgrade). Lighten up, will you?
It is quite possible that an upgrade to 7.1 is free (again, I don't know,
but m.p.vc.l frequents might).

V