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

Subject: Re: [boost] [local] Help for the Alternatives section
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2011-03-27 11:38:23

On Sun, Mar 27, 2011 at 10:04 AM, Thomas Heller
<thom.heller_at_[hidden]> wrote:
> On Saturday, March 26, 2011 11:19:46 PM Lorenzo Caminiti wrote:
>> Hello all,
>>
>> I am updating Boost.Local docs and I could use a some help in getting
>> the Alternatives section right
>>
> http://svn.boost.org/svn/boost/sandbox/local/libs/local/doc/html/boost_local/Alternatives.html
>
> I would really like to see the first row removed ...
> All alternatives you describe are using C++ syntax ... I know what you mean. But
> all the examples use regular C++ syntax.

I don't think think that Boost.Lambda and Boost.Phoenix use the usual
C++ syntax to program the "function" body. If you suggest a text
different than "Program body using C++ syntax" I am happy to consider
changing the title of the row but I think the row itself should remain
there.

This row indicates if the "function" body is programmed using the C++
syntax that programmers normally use to program C++ function bodies
(and not other C++ constructs). This also has an effect on the type of
compiler error messages that programmers will receive about their
"function" body. For example, with Boost.Local programmers will
receive the usual compiler error messages they receive when they
miss-program a function body but with Boost.Lambda and Boost.Phoenix
they will receive different compiler errors which they might be
unfamiliar with.

> Another thing what i really wonder is: You write that local classes can not be
> passed as a template parameter.
> I don't really get the difference between the local class and Boost.Local.
> What about this:
>
> int main() {
>    double sum = 0.0;
>    int factor = 10;
>
>    struct add_impl {
>        double & sum;
>        int const& factor;
>        add_impl(double sum, int factor) : sum(sum), factor(factor) {}
>
>        void operator()(double num) const
>        {
>            sum += factor * num;
>            std::clog << "Summed: " << sum << std::endl;
>        }
>    };
>
>    boost::function<void(double)> add = add_impl(sum, factor);

Your example compiles on MSVC but it does not compile on GCC. As far
as I understand it, this is not standard ISO C++ because you cannot
pass the local class as a template parameter (in this case to
boost::function).

$ g++ -Wall -Werror -I../../.. 01.cpp
01.cpp: In function ‘int main()’:
01.cpp:22: error: conversion from ‘main()::add_impl’ to non-scalar
type ‘boost::function<void ()(double)>’ requested

Can your example be made to compile on GCC and standard ISO C++?

>
>    double nums[3] = {1, 2, 3};
>    std::for_each(nums, nums + 3, add);

(A semicolumn was missing at the end of the for_each.)

>    return 0;
> }
>
> I would be happy if you sched some light on to why this isn't possible but why
> its possible with using your macros.

Boost.Local uses this trick to pass local functions as template
parameters: http://lists.boost.org/Archives/boost/2010/09/170888.php
You can see how this trick is actually implemented by the library
macros at: http://svn.boost.org/svn/boost/sandbox/local/libs/local/doc/html/boost_local/Implementation.html#boost_local.Implementation.local_functions

Thank you very much for your comments.

--
Lorenzo