Re: Returning iterator from template class

From:
Kai-Uwe Bux <jkherciueh@gmx.net>
Newsgroups:
comp.lang.c++
Date:
Wed, 31 May 2006 16:10:57 -0400
Message-ID:
<127ru2iffntkm4f@corp.supernews.com>
Jack Saalweachter wrote:

dan2online wrote:

If you want to extend the vector, inheritance is better than
composition in your case.

Eehhhhh.

It's 'riskily dangerous' to inherit from vector<T> (or any other
standard containers) because they don't have virtual destructors. No
virtual destructors mean that if you ever end up storing your
super_vector<T> as a vector<T>*, and then delete that pointer, /the
universe explodes/. Or near enough.

There are few 'never's in programming, but as a rule of thumb, I have to
stop and think real hard about extending a class with no virtual
destructor.


May I direct your attention to

  std::unary_function

or

  std::iterator

These classes are explicitly meant to be inherited from, yet they do not
have a virtual destructor.

In general, whether you need polymorphic pointers is very much dependent on
your field of programming -- I have almost no use for runtime polymorphism.
However, I do have a need to, e.g., distinguish ordinary containers from
vectors that support arithmetic operations:

  template < typename T >
  class math_vector : public std::vector< T > {
  public:

    math_vector ( void )
      : std::vector< T > ()
    {}

    template < typename A >
    math_vector ( A a )
      : std::vector< T > ( a )
    {}
      
    template < typename A, typename B >
    math_vector ( A a, B b )
      : std::vector< T > ( a, b )
    {}
      
    template < typename A, typename B, typename C >
    math_vector ( A a, B b, C c )
      : std::vector< T > ( a, b, c )
    {}
      
    template < typename A, typename B, typename C, typename D >
    math_vector ( A a, B b, C c, D d )
      : std::vector< T > ( a, b, c, d )
    {}

  }; // math_vector<>

Now, you can implement vector arithmetic along the following lines:

  template < typename T >
  math_vector< T > operator+= ( math_vector< T > & u,
                                math_vector< T > const & v ) {
    typename math_vector< T >::const_iterator v_iter = v.begin();
    for ( typename math_vector< T >::iterator u_iter = u.begin();
          u_iter != u.end();
          ++ u_iter, ++ v_iter ) {
      *u_iter += *v_iter;
    }
    return( u );
  }

and the overload does not extend to all vectors but only those that are
flagged as math_vectors in the program. Here, std::vector is used like a
policy class and its use is an implementation detail of math_vector.

It is true that you cannot use pointers to std::vector polymorphically.
Well, so just don't do that. However, to infer that inheritance should be
limited to classes with virtual destructors is an over-generalization: in
fact, policy based design uses inheritance quite extensively, yet most
policy classes have no virtual destructors.

Best

Kai-Uwe Bux

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