Re: [boost] [local] local functions ready for review

Subject: Re: [boost] [local] local functions ready for review
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2011-01-03 22:04:28

On Mon, Jan 3, 2011 at 12:58 PM, Edward Diener <eldiener_at_[hidden]> wrote:
> On 1/3/2011 12:22 PM, Lorenzo Caminiti wrote:
>>
>> Hello all,
>>
>> A few months ago, I probed for interest in a library for local
>> functions with Boost.ScopeExit's variable binding. Based on that
>> discussion, I have now implemented the Boost.Local library.
>>
>> I have uploaded the source files boost-local_0_1_0-818.zip in the
>> Boost Vault under Function Objects:
>>
>> http://www.boostpro.com/vault/index.php?&direction=0&order=&directory=Function%20Objects
>>
>> The documentation on the web:
>>
>> http://contractpp.sourceforge.net/boost-local_0_1_0-818/libs/local/doc/html/index.html
>>
>> Any comment?
>
> I can understand the potential usefulness of local functions and local
> exits, but it seems local blocks serve little purpose, other than an easier
> syntactical notice, because it is a macro, over a simple block using { }
> which can also of course access variables in the enclosing scope. I do see
> where one can bind variables in local blocks in a few ways but that seems to
> offer little over just normal C++ blocks.

It's funny that local blocks were actually the main use case that
motivated me in developing the library... My use case for local blocks
was to check assertion in a constant-correct context (this is also
explained in the Boost.Local documentation). Consider the following:

void zero(int& x) {
    x = 0;
    assert(x == 0);
}

If I mistake operator== with operator= I will get a run-time error as
the assertion will fail. However, ideally such a mistake would be
caught at compile-time if I could also program the semantic constraint
that assertions should not change the state of the program (but only
check the state of the program). So I would like to use a
"const-block" (borrowing the syntax from
http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2004/n1613.pdf
bottom of page 15):

void zero(int& x) {
    x = 0;
    const {
        assert(x == 0);
    }
}

Unfortunately, this is not valid C++ syntax :( but Boost.Local
const-binding can be used instead:

void zero(int& x) {
    x = 0;
    BOOST_LOCAL_BLOCK( (const bind)((&x)) ) {
        assert(x == 0);
    } BOOST_LOCAL_BLOCK_END
}

Now if I mistake operator== with operator= I get a compile-time error :)

This was the original use case that motivated me in developing
Boost.Local and I still think this is a good use case that shows how
local blocks can be useful. However, I agree that local functions are
by far the main feature of this library. (In fact, both local exits
and blocks essentially comes for free as they are implemented as local
functions with no parameter and not result type.)

> Nonetheless your library seems interesting. Would you consider putting it in
> the sandbox also ( or instead ) ? That way, when and if you need to update
> it, others can pick up the updates via SVN.

Yes, I am happy to upload the library in SVN. How do I do this? (I do
not have write access to Boost SVN...)

>> As far as I can tell, Boost.Local is ready for a formal review so I am
>> looking for a review manager. Any volunteer?
>
> I can understand your being eager for a review, but shouldn't you let
> interested parties use it first for a short period of time ?

Of course: Boosters please play with Boost.Local and let me know what
you think! The source is in the Boost Vault and there are quite a few
examples with the source plus a few more within the documentation
text.

I much rather have the opportunity to improve the library before a
review than having the library fail the review miserably :)

Thanks,
-- 
Lorenzo