Re: [boost] [Phoenix-3] Is Actor assignable?

Subject: Re: [boost] [Phoenix-3] Is Actor assignable?
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2011-01-22 11:01:01

Joel de Guzman wrote:

<snip>
>>>>>>>
>>>>>>> Actor a;
>>>>>>> Actor b = ...;
>>>>>>> a = b; // no-op?
>>>>>>
>>>>>> It should copy the contents. Thomas?
>>>>>
>>>>> Not really. It needs to create the assign actor (that means the
>>>>> expression template representing the assignment operation.
>>>>> It is neither a no-op nor is the assignment "executed". Remember the
>>>>> stuff about phoenix being lazy ;)?
>>>>
>>>> No! That's not the case here. There *is* still the immediate assignment
>>>> operator being called when two actors are of the same type (e.g.
>>>> when lhs and rhs are phoenix values). In these cases, a *is*
>>>> assigned to *b*. Thus, for values, the contents are copied.
>>>> Ditto for copy construction of course.
<snip>
>>
>> Seems like i don't stand a chance. I was trying to find a use case where
>> you might want the lazy version of the assignment ... couldn't find one.
>> Added the operator=(actor const&) to phoenix 3.
>>
>> Thanks for the hint.
> 
> But here's where it gets tricky: Even a slight change in type
> will make it lazy. Consider a: value<int> b: value<short>.

Ok ... we can't detect this (probably) in any sane manner.
And it is not needed to fulfill the Assignable requirement.