See below.

ublas-bounces@lists.boost.org wrote on 11/07/2005 10:49:20 PM:

> You should be a bit careful with result_of.  It does not utilize SFINAE, so
> if you write a forwarding template operator such as
>
> template <typename T, typename U>
> typename result_of<plus(T,U)>::type
> operator+(T const& x, U const& y)
> {
>    return plus()(x,y);
> }
>
> it will match ALL combinations of T,U but give a compile-time error if
> result_of<plus<T,U> > cannot be determined.  So the above usage of
> result_of turns out to be useless.  Result_of does not do any typeof magic
> either, it can determine the result_of only for function types (where the
> return type is part of the type anyway), and function objects for which a
> nested type f::result<X>::type.is defined.  So I am not sure that it can
> replace the logic of promote_traits anyway.

Not utilizing SFINAE(substitution failure is not an error) is indeed an issue if the above operator (or mechanisim in general) is fully exposed to the general user as Ian has pointed out above.  However, this negative is mitigated by two positives.  Firstly, #include <boost/utility/result_of.hpp> is a library that people can easily understand and so more people will be able to understand and contribute to ublas.  Secondly, if it is used internally where classes and types are defined for the appropriate "scalars", it does provide valuable compile-time errors because of the fact that it does not utilize SFINAE.    

Kind regards,
Andrew

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