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Inheriting from std::vector?

BCC
Hi,

A colleague has some code like this:

class CMyObject {
// Bunch of Member functions
}

class CMyObjectList: public std::vector<CMy Object>
{
// Bunch of member functions
}

I need to inherit the functionality of his object and list as well as add
some of my own:

class CMyPersonalObje ct: public CMyObject
{
// More stuff
}

class CMyPersonalObje ctList: public CMyObjectList
{
// More stuff
}

But if I pass in CMyPersonalObje ctList to a function, and then try to assign
the CMyPersonalObje ct to a local variable of that type, I get a compiler
error telling me that I cannot convert CMyObject to CMyPersonalObje ct... the
compiler thinks the list I passed in is made of the parent objects.

What is going on and how do I fix it?

Thanks,
B
Jul 19 '05
26 10749
Pete Becker wrote:
lilburne wrote:
Rolf Magnus wrote:

lilburne wrote:

However, as std::vector doesn't have a virtual destructor it
shouldn't really be being used as a base class.
There is no problem with using it as a base class as long as you don't
destroy the object through a pointer to std::vector.


And you ensure that how?

You don't do it.


I take it you mean: you don't inherit from a class that
doesn't contain a virtual destructor, not that you don't
delete a pointer to a class that doesn't have a virtual
destructor.

Jul 19 '05 #11
lilburne wrote:

Pete Becker wrote:
lilburne wrote:
Rolf Magnus wrote:
lilburne wrote:

>However, as std::vector doesn't have a virtual destructor it
>shouldn't really be being used as a base class.
There is no problem with using it as a base class as long as you don't
destroy the object through a pointer to std::vector.
And you ensure that how?

You don't do it.


I take it you mean: you don't inherit from a class that
doesn't contain a virtual destructor, not that you don't
delete a pointer to a class that doesn't have a virtual
destructor.


You take it wrong. Pay attention to context.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
Jul 19 '05 #12
Pete Becker wrote:
lilburne wrote:
Pete Becker wrote:

lilburne wrote:
Rolf Magnus wrote:

>lilburne wrote:
>
>
>
>
>>However , as std::vector doesn't have a virtual destructor it
>>shouldn 't really be being used as a base class.
>
>
>There is no problem with using it as a base class as long as you don't
>destroy the object through a pointer to std::vector.
>

And you ensure that how?
You don't do it.


I take it you mean: you don't inherit from a class that
doesn't contain a virtual destructor, not that you don't
delete a pointer to a class that doesn't have a virtual
destructor.

You take it wrong. Pay attention to context.


Amazing:

#include <iostream>
#include <memory>

using namespace std;

class A {
public:
A();
~A();
};

class B : public A {
string s;
public:
B();
~B();
};

A::A() { cout << "A::A" << endl;}
A::~A() { cout << "A::~A" << endl;}
B::B() { cout << "B::B" << endl;}
B::~B() { cout << "B::~B" << endl;}

int main()
{
auto_ptr<A> a(new B);
return 0;
}

Jul 19 '05 #13
lilburne wrote:
You take it wrong. Pay attention to context.


Amazing:


You wrote bad code, having been told it's bad. What's your point?

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
Jul 19 '05 #14
Pete Becker wrote:
lilburne wrote:
You take it wrong. Pay attention to context.


Amazing:

You wrote bad code, having been told it's bad. What's your point?


What was bad about it?

Jul 19 '05 #15
lilburne wrote:

Pete Becker wrote:
lilburne wrote:
You take it wrong. Pay attention to context.
Amazing:

You wrote bad code, having been told it's bad. What's your point?


What was bad about it?


PLONK.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
Jul 19 '05 #16
Pete Becker wrote:
lilburne wrote:
Pete Becker wrote:

lilburne wrote:
>You take it wrong. Pay attention to context.
>

Amazing:

You wrote bad code, having been told it's bad. What's your point?


What was bad about it?

PLONK.


Tsk! You jump in to sanction using a class inheritance
design, that not only restricts how the heirarchy may be
used, but which also has the potential to cause resource
leakage, and then become all sniffy.

Its no good simply saying 'don't do it', because it will be
done. If you took 10 'C++' programmers at random and asked
"What does the following program print?" you'd be lucky to
get the correct answer from 4 of them. Ask the 4 that got it
right the same question again having removed virtual from
A's destructor and you might get 2 left.

class A {
public:
virtual void func1() { cout << "a::func1" << endl; }
void func2() { cout << "a::func2" << endl; }
virtual void func3() { cout << "a::func3" << endl; }
virtual ~A() { cout << "a::~a" << endl; }
};

class B {
public:
virtual void func1() { cout << "b::func1" << endl; }
void func2() { cout << "b::func2" << endl; }
void func3() { cout << "b::func3" << endl; }
virtual ~B() { cout << "b::~b" << endl; }
};

int main()
{
B* b = new B;
A* a = b;
a->func1();
a->func2();
a->func3();

b->func1();
b->func2();
b->func3();

delete a;
return 0;
}

Its astonishing how many programmers do not understand the
import of the keyword virtual. But you know that.

Jul 19 '05 #17
On Fri, 24 Oct 2003 21:58:53 +0100, lilburne <li******@godzi lla.net> wrote:
Pete Becker wrote:
lilburne wrote:
You take it wrong. Pay attention to context.
Amazing:

You wrote bad code, having been told it's bad. What's your point?


What was bad about it?


You're destroying a non-polymorphic class object polymorphically (through a
pointer to the base class object).

But you knew that.

It's illogical to ban usage of a feature just because that feature _can_
be abused, since almost all language features _can_ be abused, and I think
your use of such an illogical argument was the reason why Pete plonked you.

Jul 19 '05 #18
Alf P. Steinbach wrote:
On Fri, 24 Oct 2003 21:58:53 +0100, lilburne <li******@godzi lla.net> wrote:

Pete Becker wrote:

lilburne wrote:
>You take it wrong. Pay attention to context.
>

Amazing:

You wrote bad code, having been told it's bad. What's your point?


What was bad about it?

You're destroying a non-polymorphic class object polymorphically (through a
pointer to the base class object).

But you knew that.

It's illogical to ban usage of a feature just because that feature _can_
be abused, since almost all language features _can_ be abused, and I think
your use of such an illogical argument was the reason why Pete plonked you.


It is unlikely that Pete Becker would derive a class from
a base that has a non-virtual destructure, without some
overriding reason to do so. Because he knows it is a
resource leak waiting to happen, and that safer alternatives
are usually available.

To pretend that you can ensure that the polymorphic deletion
on such a class heirarchy can be avoid just by saying "Don't
do it" is ridiculous. The OP not only has an inheritance
tree of depth four, but is also inheriting from a class that
already exists. As there are already functions in the system
that process the base class polymorphism is going to occur.

Certain language features ought to be used with caution, IMO
one should learn how to use them properly before you start
to abuse them.

Jul 19 '05 #19
On Sat, 25 Oct 2003 02:55:25 +0100, lilburne <li******@godzi lla.net> wrote:
Certain language features ought to be used with caution, IMO
one should learn how to use them properly before you start
to abuse them.


Well, modest that I am I find it unlikely that either Pete or I don't
know how to use inheritance properly, and I find it unlikely that either
of us would abuse the mechanism.

Perhaps you're referring to the OP?

But the OP didn't code inheritance from std::vector; he's, er, inherited
that code...

Perhaps, then, you're referring to your own abusive example code?

Jul 19 '05 #20

This thread has been closed and replies have been disabled. Please start a new discussion.

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