[boost] Boost.Lambda

From: Justinas V.D. (vygintas.daugmaudis_at_[hidden])
Date: 2004-04-29 14:05:19

Hello,

Perhaps it's just me being inattentive or Boost.Lambda lacks some 
feature I would consider quite useful. Rummaging through documentation 
and source code did not turn out to be constructive.

Well, the problem is:

struct foo
{
    template<typename ExprT>
    typename ExprT::result_type operator[](ExprT expr)
    {
          return expr(/*params here*/);
    }
};

Umm... We can write as follows then:

foo bar;

bar[
    some_stl_conformant_unary_functor() // or binary, or whatever... 
compiles OK
];

It would be nice to write little lambda expressions too,
bar[
    std::cout << _1  // Fails  miserably
];
for example.

The problem is that lambda expressions do not have result_type defined 
although technically they are functors too (like STL functors). It would 
be trivial to write little metafunction wrapper though. After all, 
boost::lambda::is_lambda_functor metafunction exists but I haven't found 
lambda_result_type traits or something like that. Documentation 
conveniently skips through that part of implementation and code is a bit 
obscure at times.

Is there a way to implement such a metafunction

    boost::lambda::functor_result<T>::type that it would return result 
type of lambda expression if T is lambda
    expression or T::result_type if T is ordinary STL functor, or just 
null_type if neither of them?

that its implementation would not turn out to be a pain in the... emm... 
neck? Or perhaps this could be a part of Boost.Lambda library?

Regards,
Justinas V.D.