AMDG Andrey Semashev wrote:
On 03/16/2010 02:02 AM, Steven Watanabe wrote:
Andrey Semashev wrote:
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.
Then I consider your dispatcher interface to be the unsafe thing. Is there a good reason to allow the attribute value to be constructed on the fly every time it's needed? The only built-in attribute that does this is the current_thread_id attribute, and I don't see a problem with saving the current thread id when getting the attribute's value. In Christ, Steven Watanabe