Hi, Brian replied to this already, and pointed out that what you want is the result_of template. That's not yet part of BLL. There's a similar mechanism though (sig template) that is explained in section 5.3.4. Note that just the std result_type typedef is not an adequate mechanism for expressing the return type of a lambda functor (which is polymorphic). See below...
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*/); } };
If ExprT is a lambda functor, say -_1, the return type is not detemined by ExprT alone, but rather by ExprT and the type of the argument with which the lambda functor is called. float f; int i; (-_1)(f) // returns float (-_1)(i) // returns int Jaakko
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.
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