Re: Inherite form stl container classes

From:
Kai-Uwe Bux <jkherciueh@gmx.net>
Newsgroups:
comp.lang.c++
Date:
Mon, 02 Jun 2008 16:18:23 -0400
Message-ID:
<48445590$0$25952$6e1ede2f@read.cnntp.org>
ademirzanetti wrote:

On Jun 2, 4:33 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:

ademirzanetti wrote:

I would like to listen your opinions about inherit from a STL class
like list. For example, do you think it is a good approach if I
inherit from list to create something like "myList" as in the example
below ?

#include "Sector.h"

using namespace boost;

typedef std::list<Setor>::iterator ListaSetorIterator;
typedef std::list<Setor>::const_iterator ListaSetorConstIterator;

class SectorList: private std::list<Sector> {
public:

        using std::list<Sector>::begin;
        using std::list<Sector>::end;

        SectorList();
        ~SectorList();

        result add( const Sector& );
        result modify( const Sector& );
        result get( Sector&, const uint32_t ) const;
        result remove( const uint32_t );
private:
        using std::list<Sector>::push_back;
};

I do not see a problem on it. I believe it is better to inherit in
this way than try to create my own list from scratch, but I also want
to listen more opinions about it.

Thanks in advance ;-)


I am not sure what it is you're expecting us to tell you. Private
inheritance is a method of implementing your class in terms of the other
class. You're essentially creating an Adapter (see Adapter pattern, Goa
book or on the web). Another way to program an Adapter in C++ is
containment.

You've followed one of the two ways commonly accepted, why wouldn't it
be a good approach?

I suppose you're not expecting us to criticise your code on its own
merits, like the degree of completeness of the interface without any
given requirements, or typos, or extraneousness of some declarations...

V
--
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I do not respond to top-posted replies, please don't ask


Actually, the questions is, Is there any issue inheriting from STL
classes ?
I read some papers which say that the "STL classes were not created to
be inherited", but they did not mention the reasons. They says that
there are some pitfalls when it is used in this way that are hard to
avoid.


a) That paper probably referred to public inheritance. Private inheritance
from STL containers does not present any problems.

b) As for public inheritance from STL container, there are two commonly
mentioned problems:

b1) STL containers do not have virtual destructors.

This is not really a problem. Just don't delete pointers to the derived
classes through pointers of the base class (and why would you have any use
for such polymorphic container pointers in the first place). It's about as
problematic in practice as inheriting from std::iterator.

b2) Functions of the type

  template < typename T >
  std::vector<T> reverse ( std::vector<T> const & v );

will match any object derived from std::vector but usually have the wrong
return type.

This is a more serious objection. However, it is not at all clear whether
inheritance from std::vector is to blame or whether the function reverse is
just ill-designed. Maybe, it should be:

  template < typename Sequence >
  Sequence reverse ( Sequence const & v );

possibly enriched by some template magic to make sure it only matches
sequences. Therefore, it depends by and large on your code base whether
this problem is easy to avoid or not. If you have functions like reverse
whose signatures have been frozen, it can be cumbersome to derive from
container classes.

With public inheritance from standard containers, you have to be aware of
the above caveats. If you are (and you are confident that the maintenance
programmers who will have to deal with your code are, too), then inheriting
publicly from standard containers can be justified in certain cases. E.g.,
if you are doing linear algebra, you might want to overload operator+ to do
element wise addition. It would be a bad idea to just dump that overload
into global namespace for all vectors (e.g., it might interfere with the
idea of someone else to have operator+ denote concatenation of sequences).
In that case, a quick

  template < typename ArithmeticType >
  struct arithmetic_vector : public std::vector< ArithmeticType > {
    // some constructors
  };
  
  template < typename ArithmeticType >
  arithmetic_vector< ArithmeticType >
  operator+ ( arithmetic_vector< ArithmeticType > const & lhs,
              arithmetic_vector< ArithmeticType > const & rhs ) {
    assert( lhs.size() == rhs.size() );
    ...
  }

can serve as a templated typedef that does not create just an alias but a
true independent type (that will convert transparently to std::vector in
cases needed and with the right constructor, conversion the other way
around is also no problem). However, private inheritance with a complete
set of forwarding methods is considered cleaner by many.

A final real issue with public inheritance from std::vector is the
introduction of additional invariants (e.g., the condition that the sum of
all elements be 0). That cannot work since the underlying base class allows
client code to invalidate the invariant.

Again, I always used it in this way for years without problems and I
just expected to listen if someone already had any issues with it.


As Victor said, there are no issues with private inheritance from standard
containers.

Best

Kai-Uwe Bux

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