Re: [boost] Re: Querying usefulness of a macro for inheriting fromSTLcontainers

From: christopher diggins (cdiggins_at_[hidden])
Date: 2005-02-27 13:11:11

----- Original Message ----- 
From: "Jeff Garland" <jeff_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Saturday, February 26, 2005 9:55 PM
Subject: Re: [boost] Re: Querying usefulness of a macro for inheriting 
fromSTLcontainers

> On Fri, 25 Feb 2005 18:26:22 +0000, Jonathan Wakely wrote
>
>> > >christopher diggins wrote: I think publicly inheriting from a class 
>> > >that
>> > >isn't designed for inheritance is a bad idea. There are type safety
>> > >problems, non-virtual destructors, slicing etc.
>> >
>> > Hi Daniel thanks for the help and suggestions ( and Kevin and Jonathan 
>> > as
>> > well),
>> >
>> > I have heard here and there, inheriting from STL container is a "bad 
>> > idea",
>> > but I don't specifically know all the reasons why. I am not an expert 
>> > in
>> > this area (heck or any part of C++ for that matter). Isn't it only bad 
>> > if
>> > someone tries to delete the object using a base class pointer?
>>
>> That's one reason, yes.
>>
>> (btw, this is Item 35 in Sutter and Alexandrescu's C++ Coding
>> Standards)
>
> At the risk of going against the entrenched dogma here, I'd suggest there 
> are
> significant cases where this is a good and useful thing to do. I think the
> original guidance came from Scott Meyers Effective C++ #14 -- make 
> destructors
> virtual in base classes.  The problem I have with this is that I find 
> there
> are plenty of situations where I might want to add some convenience 
> functions
> to an stl container in which the subclass has a trivial destructor (eg: no
> allocated resources)

I am a little confused here, does MySubClass below have a "trivial" 
destructor?

  MySubClass : MyBaseClass {
    int x;
  };

In other if I write:

  MyBaseClass* p = new MySubClass;
  delete p;

Do I leak the memory for integer x? And then what happens here:

  Baz {
    ~Baz() { cout << "destroying baz"; }
  };

  MySubClass : MyBaseClass {
    Baz x;
  };

  MyBaseClass* p = new MySubClass;
  delete p;

Does the baz get destroyed? I always assumed it should.

>> If you want to inherit from, say, vector, do you _really_ want _all_
>> its public member functions?
>
> Yes, I frequently do.
>
> Jeff

So Jeff, would you find a set of macros for facilitating the inheritance of 
STL containers useful?

Christopher Diggins
Object Oriented Template Library (OOTL)
http://www.ootl.org