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Subject: Re: [boost] Re : [local_function] any interest in a LocalFunction library?
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2010-08-23 08:07:19

On Mon, Aug 23, 2010 at 7:48 AM, Pierre Morcello
<pmorcell-cppfrance_at_[hidden]> wrote:
> Lorenzo Caminiti wrote :
>> Is there interest in a library that implement local
>> functions for C++?
> One year ago, I made a proposition for a boost.breakable. Some people were interested in the library, if I could widen it a little. David Abraham & Vicente Botet expressed interest in a macro local function that would work like scope exist.
> So I wrote a first implementation. But I have not posted back on this subject, because I was unsure about the macro interface, since a few variations on its interface (ex: passing this or not) could be done.
> Here I send 2 little examples of my tests. The 'V3' is almost the same as the one you posted. The 'V4' is a try wich uses only 1 macro call. Unfortunately, these 2 use currently __VA_ARGS__, and were only tested on visual 2008.
> But yes, there is an interest in a local function.

I can implement Boost.LocalFunction macros, which are similar to your
'V3' macros, in pure C++ ISO standard (no __VA_ARGS__). I will study
your code more.

On Sun, Aug 22, 2010 at 9:27 PM, Jeffrey Lee Hellrung, Jr.
<jhellrung_at_[hidden]> wrote:
> On 8/22/2010 6:01 PM, OvermindDL1 wrote:
>> On Sun, Aug 22, 2010 at 11:18 AM, Mathieu -<ptr.jetable_at_[hidden]>  wrote:
>>> I might be totally wrong here but I don't see how it's different from
>>> lambdas...
>> What can it do differently from boost::phoenix?  I am guessing you
>> just create and instance a struct right there, but boost::phoenix
>> would be more succinct.
> Except Boost.Phoenix isn't as expressive as "real" C++, is it?  E.g., local
> variables are somewhat clumsy (opinion) in the current incarnation of
> Boost.Phoenix.
> Also, it seems reasonable to implement BOOST_LOCAL_FUNCTION in terms of
> lambdas if they exist, but perhaps one would want to use local functions on
> non-lambda-enabled compilers.

I am NOT an expert of Boost.Phoenix and Boost.Lambda so *please
correct me if I am wrong*.

I think the main different between using Boost.Phoenix or Boost.Lambda
and using Boost.LocalFunction is that in Boost.LocalFunction the local
function body completely retains C++ syntax (a part from using `this_`
instead of `this`... which is not ideal...). Now this might or it
might not be a problem. For example, in my Boost.Contract application
I need the users to write the local function body and pass it to a
macro so it is a requirement for that body to be written in usual C++
because I cannot request Boost.Contract users to learn Boost.Phoenix
or Boost.Lambda. However, for other applications using Boost.Phoneix
or Boost.Lambda might be fine.

I think C++0x lambdas will be a valid option in replacing
Boost.LocalFunction but they are only part of *new* C++ standard and
not supported by all compilers.


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