[boost] Re: filter_iterator requires copy-constructible predicate?

From: troy d. straszheim (troy_at_[hidden])
Date: 2004-12-01 09:51:29

I hit send and thought maybe that wasn't completely clear, sorry about
that.  Here's a use-case:

  //
  // pure virtual base class
  //
  struct int_predicate 
  {
    virtual ~int_predicate(){};
    virtual bool operator()(int x) = 0;

  protected:
    int_predicate(){};
  private:
    int_predicate(const int_predicate&);
    int_predicate& operator=(const int_predicate&);
  };
  
  struct is_positive_number : public int_predicate 
  {
    virtual bool operator()(int x) { return x > 0; }
  };

  array<int, 10> a = { 0, -1, 2, -3, 4, -5, 6, -7, 8, -9 };

  is_positive_number pos;

  int_predicate &ispos_ref = pos;

  for (filter_iterator<int_predicate,array<int,10>::iterator> iter = 
         make_filter_iterator<int_predicate>(ispos_ref, a.begin(), a.end());
       iter.base() != iter.end();
       iter++)
    {
      // whatever
    }

troy d. straszheim writes:
 > 
 > Hi boost,
 > 
 > The filter_iterator as implemented disallows creating iterators via
 > reference-to-base, as it requires its Predicate objects to be
 > copy-constructible.  This is a problem for a class that is passed a
 > ref to a predicate from elsewhere.  It's a very easy fix, though, I'm
 > wondering what the rationale originally was for the restriction.  If
 > it isn't significant, I'd propose the change below, as (at least from
 > my narrow perspective) it is a big plus to be able to do this.
 > 
 > 
 > 56c56
 > <     filter_iterator(Predicate& f, Iterator x, Iterator end = Iterator())
 > ---
 > >       filter_iterator(Predicate f, Iterator x, Iterator end = Iterator())
 > 82c82
 > <     const Predicate& predicate() const { return m_predicate; }
 > ---
 > >       Predicate predicate() const { return m_predicate; }
 > 106c106
 > <     Predicate& m_predicate;
 > ---
 > >       Predicate m_predicate;
 > 112c112
 > <   make_filter_iterator(Predicate& f, Iterator x, Iterator end = Iterator())
 > ---
 > >   make_filter_iterator(Predicate f, Iterator x, Iterator end = Iterator())
 > 
 > 
 > Thanks for your time,
 > 
 > -t

-- 
troy d. straszheim                                         
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