Re: [boost] [local] Help for the Alternatives section

Subject: Re: [boost] [local] Help for the Alternatives section
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2011-03-27 11:35:46

On Sunday, March 27, 2011 05:43:39 PM Lorenzo Caminiti wrote:
> On Sun, Mar 27, 2011 at 10:07 AM, Thomas Heller
> <thom.heller_at_[hidden]> wrote:
> >> On a separate note, is there any way I can prevent the following
> >> `factor = 0` from compiling (e.g., making factor a const within the
> >> Phoenix function expression)?
> >>
> >> #include <boost/spirit/include/phoenix.hpp>
> >> #include <iostream>
> >> #include <vector>
> >> #include <algorithm>
> >>
> >> int main() {
> >>     double sum = 0.0;
> >>     int factor = 10;
> >>
> >>     std::vector<double> v(3);
> >>     v[0] = 1.0; v[1] = 2.0; v[2] = 3.0;
> >>
> >>     std::for_each(v.begin(), v.end(), (
> >>         factor = 0, // <==== Can I prevent this with a compiler error?
> >
> > No, because this is just plain old C++. assigning zero to a value ... there 
is
> > nothing what phoenix can do here.
> 
> Ok, I understand. Thanks.
> 
> > The guy who wrote the code could have said: const int factor = 10; 
assignment
> > would then be impossible^W not allowed.
> 
> Yes, but the use case would be to have factor const *only* within the
> "function" passed to for_each while keeping it mutable in the
> enclosing scope. Therefore, programmers have to declare factor
> not-const within main() and I was wondering if there was a way using
> Phoenix to add the const only locally within the "function" passed to
> for_each (this is done by Boost.Loccal using "constant-binding" as in
> `const bind& factor`).

Oh right, you can do that ...:

let(_a = ref(factor))[...]; // <-- bind as non-const reference
let(_a = cref(factor))[...]; // <-- bind as const reference

Error messages are a mess currently ... but can be improved. Consider it as a 
bug.