Re: [boost] [log] Comments

Subject: Re: [boost] [log] Comments
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2010-03-16 11:34:32

On 03/16/2010 02:02 AM, Steven Watanabe wrote:
>
> Andrey Semashev wrote:
>> On 03/15/2010 11:22 PM, Vladimir Prus wrote:
>>> The corresponding code using lambda would be, presumably:
>>>
>>> flt::attr< ...>("System") == m_sys
>>>
>>> Would it be possible to make the following work in the first case:
>>>
>>> if (attrs["System"] != m_sys)
>>> return false;
>>
>> It doesn't specify the type of the value.
>
> You can deduce the type from the type of m_sys.

Ah, right. Clever trick.

>>> ? Or, if changing type/behaviour of operator[] is undesirable, what
>>> about
>>>
>>> if (attrs->get_nothrow("System") != m_sys)
>>> return false;
>>
>> That should be:
>>
>> if (attrs.get_nothrow< System >("System") != m_sys)
>> return false;
>>
>> Yes, it could be done. But as stated elsewhere in the discussion, this
>> would require System to be copyable (which might be fine in this
>> case). And it doesn't really differ from:
>>
>> bool result = false;
>> extract< System >("System", attrs, var(result) = _1 != m_sys);
>> return result;
>
> except that the former is more readable.
>
>> but with extract it doesn't have the copyability restriction.
>
> Why does this require System to be copyable? Can't you
> return a reference?

Returning a reference looks like an unsafe interface design to me. 
Remember that double dispatch is taken place here, so that the caller's 
visitor is invoked by the attribute value, with a reference to the 
actual value. In order to return this reference from get_nothrow, it has 
to be saved by the visitor and restored after returning from the 
attribute value dispatch method. At this point the reference may 
potentially be broken.