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

27 Feb 2005

      ----- Original Message ----- 
From: "Jeff Garland" <jeff@crystalclearsoftware.com>
To: <boost@lists.boost.org>
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