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.