Inheriting from std::vector?

**lilburne** · Jul 19 '05, 07:36 PM

Re: Inheriting from std::vector?

BCC wrote:
[color=blue]
>
> 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?
>[/color]

Would need to see the code where the assignment is made.
However, as std::vector doesn't have a virtual destructor it
shouldn't really be being used as a base class. That isn't
your actual problem though.

**Alf P. Steinbach** · Jul 19 '05, 07:36 PM

Re: Inheriting from std::vector?

On Thu, 23 Oct 2003 23:47:14 GMT, "BCC" <bryan@akanta.c om> wrote:
[color=blue]
>A colleague has some code like this:
>
>class CMyObject {
> // Bunch of Member functions
>}[/color]

Missing semicolon.

[color=blue]
>class CMyObjectList: public std::vector<CMy Object>
>{
> // Bunch of member functions
>}[/color]

Ditto.

Without further information this inheritance is a little suspicious.

I can thing of good reasons to inherit from std::string, but not std::vector.

[color=blue]
>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,[/color]

There is no relationship between CMyPersonalObje ctList and CMyPersonalObje ct
in what you have shown, so the above seems meaningless.

[color=blue]
> I get a compiler
>error telling me that I cannot convert CMyObject to CMyPersonalObje ct...[/color]

The compiler is right, you cannot do that unless you provide a conversion.

[color=blue]
> the compiler thinks the list I passed in is made of the parent objects.[/color]

Where on earth did you get that absurd idea? Compilers don't think.

[color=blue]
>What is going on[/color]

See e.g. <url: http://news.google.com/>.

[color=blue]
>and how do I fix it?[/color]

Perhaps follow the advice of Michael Moore?

**Grzegorz Sakrejda** · Jul 19 '05, 07:36 PM

Re: Inheriting from std::vector?

On Thu, 23 Oct 2003 23:47:14 GMT, BCC <bryan@akanta.c om> wrote:
[color=blue]
> 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
>
>
>[/color]

std::vector is a template . CMyObjectList should also be a template, at
least if you want to have personalobject list.

template <class T> CObjectList : public std::vector<T> {
some more
};

typedef CObjectList<CMy PersonalObject> CMyPersonalObje ctList;

--
grzegorz

**Mike Wahler** · Jul 19 '05, 07:37 PM

Re: Inheriting from std::vector?

"Alf P. Steinbach" <alfps@start.no > wrote in message
news:3f986ad9.5 08185656@News.C IS.DFN.DE...[color=blue][color=green]
> > the compiler thinks the list I passed in is made of the parent objects.[/color]
>
> Where on earth did you get that absurd idea? Compilers don't think.[/color]

Understatement of the year. :-)

-Mike

**Davlet Panech** · Jul 19 '05, 07:37 PM

Re: Inheriting from std::vector?

"BCC" <bryan@akanta.c om> wrote in message
news:6MZlb.9971 $QQ.83515602@ne wssvr21.news.pr odigy.com...[color=blue]
> 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[/color]
assign[color=blue]
> 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...[/color]
the[color=blue]
> compiler thinks the list I passed in is made of the parent objects.[/color]

It won't work, because the vector contains CMyObjects, not
CMyPersonalObje cts. So whenever you insert a CMyPersonalObje ct in that
vector, it'll get "truncated" to it's base type. You might be able to use a
vector of pointers to (dynamically-allocated) CMyObjects (+derivatives)
instead, but you'll run into other problems:
- Manual memory allocation is difficult and error-prone (but "smart pointer"
libraries might help, see for example www.boost.org )
- You'll have to downcast the stored addresses all the time
- I think it is wrong to have CMyPersonalObje ctList as a subclass of
CMyObjectList, because such hierarchy permits insertions of CMyObjects into
CMyPersonalObje ctList (you might want to do a search on "Liskov Substitution
Principle" to learn why this is a bad idea. For example read this article:
http://www.brent.worden.org/articles...rinciple.html).

What exactly are you trying to do? Maybe there's a better way to do it.
[color=blue]
>
> What is going on and how do I fix it?
>
> Thanks,
> B
>
>[/color]

**David Rubin** · Jul 19 '05, 07:37 PM

Re: Inheriting from std::vector?

Mike Wahler wrote:[color=blue]
> "Alf P. Steinbach" <alfps@start.no > wrote in message
> news:3f986ad9.5 08185656@News.C IS.DFN.DE...
>[color=green][color=darkred]
>>>the compiler thinks the list I passed in is made of the parent objects.[/color]
>>
>>Where on earth did you get that absurd idea? Compilers don't think.[/color]
>
>
> Understatement of the year. :-)[/color]

OP must have confused "The Compiler That Thinks" with "The Thinking
Man's Compiler."

/david :-)

--
"As a scientist, Throckmorton knew that if he were ever to break wind in
the echo chamber, he would never hear the end of it."

**Rolf Magnus** · Jul 19 '05, 07:37 PM

Re: Inheriting from std::vector?

lilburne wrote:
[color=blue]
> However, as std::vector doesn't have a virtual destructor it
> shouldn't really be being used as a base class.[/color]

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.

**lilburne** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

Rolf Magnus wrote:
[color=blue]
> lilburne wrote:
>
>[color=green]
>>However, as std::vector doesn't have a virtual destructor it
>>shouldn't really be being used as a base class.[/color]
>
>
> 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.
>[/color]

And you ensure that how?

**Pete Becker** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

lilburne wrote:[color=blue]
>
> Rolf Magnus wrote:
>[color=green]
> > lilburne wrote:
> >
> >[color=darkred]
> >>However, as std::vector doesn't have a virtual destructor it
> >>shouldn't really be being used as a base class.[/color]
> >
> >
> > 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.
> >[/color]
>
> And you ensure that how?[/color]

You don't do it.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)

**lilburne** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

Pete Becker wrote:
[color=blue]
> lilburne wrote:
>[color=green]
>>Rolf Magnus wrote:
>>
>>[color=darkred]
>>>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.
>>>[/color]
>>
>>And you ensure that how?[/color]
>
>
> You don't do it.
>[/color]

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.

**Pete Becker** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

lilburne wrote:[color=blue]
>
> Pete Becker wrote:
>[color=green]
> > lilburne wrote:
> >[color=darkred]
> >>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?[/color]
> >
> >
> > You don't do it.
> >[/color]
>
> 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.[/color]

You take it wrong. Pay attention to context.

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)

**lilburne** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

Pete Becker wrote:
[color=blue]
> lilburne wrote:
>[color=green]
>>Pete Becker wrote:
>>
>>[color=darkred]
>>>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.
>>>[/color]
>>
>>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.[/color]
>
>
> You take it wrong. Pay attention to context.
>[/color]

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;
}

**Pete Becker** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

lilburne wrote:[color=blue]
>[color=green]
> > You take it wrong. Pay attention to context.
> >[/color]
>
> Amazing:
>[/color]

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

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)

**lilburne** · Jul 19 '05, 07:38 PM

Re: Inheriting from std::vector?

Pete Becker wrote:
[color=blue]
> lilburne wrote:
>[color=green][color=darkred]
>>>You take it wrong. Pay attention to context.
>>>[/color]
>>
>>Amazing:
>>[/color]
>
>
> You wrote bad code, having been told it's bad. What's your point?
>[/color]

What was bad about it?

Inheriting from std::vector?

Inheriting from std::vector?

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