From: Andrew Rieck (arieck_at_[hidden])
Date: 2005-07-11 20:01:43
ublas-bounces_at_[hidden] wrote on 11/07/2005 10:49:20 PM:
> You should be a bit careful with result_of. It does not utilize SFINAE,
> 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
> either, it can determine the result_of only for function types (where
> return type is part of the type anyway), and function objects for which
> 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.
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