Re: Question about objects

From:

LR <lruss@superlink.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 29 Aug 2007 17:21:51 -0400

Message-ID:

<46d5e315$0$26697$cc2e38e6@news.uslec.net>

R2D2 wrote:

Your top posting has been moved. This will not be posted in comp.lang.c.

On 29 Aug 2007 at 0:01, LR wrote:

R2D2 wrote:

Hi,

could someone please explain to me, why in the
following code the second approach does not work?

Thanks for your help.

#include <vector>

class Base {};

class Inherited : public Base{};

Inherited is a kind of Base;
http://www.parashift.com/c++-faq-lite/basics-of-inheritance.html#faq-19.3

But

std::vector<Inherited> is not a std::vector<Base>
http://www.parashift.com/c++-faq-lite/proper-inheritance.html#faq-21.3

> I don't really follow this - could you give a fuller explanation?

I'll take a stab at it, but I suggest that you read the faq and ask a
more specific question.

Also, please don't post your follow up to comp.lang.c. This doesn't
belong there.

Below, Inherited is a child class of Base and inherits from Base.
X<Inherited> is not a child class of X<Base> and does not inherit from
X<Base>

Compare this with Y<Inherited> and Y<Base> below.

class Base {
public:
     virtual ~Base() {}
};

class Inherited : public Base {};

template<typename T>
class X {};

template<typename T>
class Y : public Base {}

int main() {

     // ok
     Inherited i;
     Base *pb = &i;
     Base &b = i;

     X<Inherited> xi;
     X<Base> *pxb = ξ // not ok, won't compile
     X<Base> &xb = xi; // not ok, won't compile

     // ugly, but both Y<Inherited> and Y<Base>
     // inherit from Base
     // It's just an example.
     Y<Inherited> yi;
     Y<Base> yb;
     Base &pyi = yi;
     Base &pyb = yb;

}

LR