Re: [boost] Interest in breakable labeled scope emulation?

Subject: Re: [boost] Interest in breakable labeled scope emulation?
From: TONGARI (tongari95_at_[hidden])
Date: 2013-01-16 07:20:48

2013/1/16 Vicente J. Botet Escriba <vicente.botet_at_[hidden]>

> Le 15/01/13 18:44, Jeffrey Lee Hellrung, Jr. a écrit :
>
>  On Sun, Jan 13, 2013 at 4:30 AM, TONGARI <tongari95_at_[hidden]> wrote:
>>
>>  Hi all,
>>>
>>> One of the few things I appreciate in Java is the labeled-break feature.
>>> I think C++ would provide it as well in future standard but it seems that
>>> there's no even such a proposal.
>>>
>>> In lack of the language feature, a simple emulation can be used:
>>>
>>> ------------------------------**---------------------
>>> #define BOOST_SCOPE(name) \
>>> if (const bool LABELED_SCOPE_##name = false){break(name); name:;} else
>>>
>>>
>>> #define break(name) \
>>> (void)LABELED_SCOPE_##name; goto name;
>>> ------------------------------**---------------------
>>> Now we can write:
>>>
>>> BOOST_SCOPE(a)
>>> {
>>>      break(a);
>>>      cout << "123\n";
>>> }
>>>
>>> The real world usage would reside in nested loop&switch where
>>> labeled-break
>>> really shines.
>>>
>>> Thoughts? Sorry if this idea is too simple and somewhat rejected
>>> before...
>>>
>>>  It's a nice thought but:
>> (a) I think the situation where you want to break and/or continue more
>> than
>> one loop-level out at a time is rare; and
>>
> Agree and its use /could/ be a sign that a refactoring is needed, but not
> always.
>
>  (b) in those rare situations, explicitly using gotos and labels (with a
>> descriptive name) is reasonably brief and readable;
>> so I would be hesitant to adopt this.
>>
>>
> When we use label/goto directly there is a risk that the user goto to the
> label without constructing all the scoped variables, v in this case.
>
> if (cnd3) goto L; // bad as v constructor will not be called.
> {
>   T v;
>   for(;;)
>   {
>     while (cnd)
>     {
>       if (cnd2) goto L; // go to [1]
>     }
>   }
>   L: // [1]
>   std::cout << v <<std::endl;
> }
>

Actually this won't compile if T is not a POD.