Re: [Boost-users] Problems using mpl::if_ and function_types::result_type

Subject: Re: [Boost-users] Problems using mpl::if_ and function_types::result_type
From: Max Vasin (max.vasin_at_[hidden])
Date: 2011-11-24 09:26:46

Hello, Allan, although I do not know the answer I post some results
that I have got. Your problem has nothing to do with either MPL or
function_types.
First of all declaring function to take a void parameter is
deprecated, the type `void (void)' is in fact `void ()'.  When you
declare a function type void (R) where R
depends on a template parameter compiler expects to get an unary
function type but when R is void the function is nullary (and turning
an unary function into
nullary seem to be forbidden). This minimal example demonstates the issue:

template <typename F>
struct A {
  typedef void type(F);
};

int main(int argc, const char *argv[])
{
  A<int> ok;
  A<void> fail;
}

What you need is partial template specialization. And I will wait for
the expert answer.

-- 
WBR,
Max Vasin.