Re: [boost] Re: concept archetypes

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-10-16 21:51:58

On Wednesday 16 October 2002 10:17 pm, Kevin S. Van Horn wrote:
> Douglas Gregor writes:
> > there are yet more subtle errors (this one from the Comeau standard
> > library, libcomo) that aren't caught by the concept checks library:
> >
> >  template <class _InputIter, class _Predicate>
> >  inline _InputIter find_if(_InputIter __first, _InputIter __last,
> >                            _Predicate __pred,
> >                            input_iterator_tag)
> >  {
> >    while (__first != __last && !__pred(*__first))
> >      ++__first;
> >    return __first;
> >  }
>
> OK, I'm stumped.  What's wrong with the above code?  (The GNU libstdc++
> code is identical.)

InputIterators are required to be EqualityComparable, which states that the 
result of '__first != __last' should be convertible to bool. Similarly, a 
(unary) Predicate is required to be a function object whose result_type is 
convertible to bool. There is no requirement that the expression 
  __first != __last && !__pred(*__first)
be valid, so we _could_ have the pathological case where:

__first != __last returns an object of type 'Foo' (Foos are convertible to 
bool)
__pred(*__first) returns an object of type 'Bar' (Bars are convertible to 
bool)
operator&&(Foo, Bar) is defined with different semantics than operator&&(bool, 
bool)

The fix is just a pair of static_cast<bool>'s, and it's doubtful that this 
_particular_ error would ever occur, but still it's an error that is 
non-obvious and is not found by the Boost Concept Check library (one would 
need to poison operator&&(boolean_archetype, boolean_archetype to find it). 

        Doug