Re: [boost] [Bind] Interoperability of bind/mem_fn with transform_iterator

Subject: Re: [boost] [Bind] Interoperability of bind/mem_fn with transform_iterator
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-05-04 23:59:53

On 05/04/2010 06:20 PM, Daniel Walker wrote:
> On Tue, May 4, 2010 at 6:55 PM, Jeffrey Lee Hellrung, Jr.
> <jhellrung_at_[hidden]>  wrote:
>> On 5/4/2010 3:49 PM, Daniel Walker wrote:
>>>
>>> On Tue, May 4, 2010 at 1:49 PM, Peter Dimov<pdimov_at_[hidden]>    wrote:
>>>>
>>>> Samuel Debionne:
>>>>>
>>>>> Anyway, in my opinion, using bind/mem_fn/lambda with transform_iterator
>>>>> is a common use case. That would be great to have an option to add a
>>>>> default constructor in those libs... or is it to risky ?
>>>>
>>>> The proper fix would be to make transform_iterator not require a
>>>> default-constructible function object, by using boost::optional or an
>>>> equivalent.
>>>
>>> That's a great idea! I took a look at it, and the required changes
>>> seem to be minor. I made a ticket and attached a patch.
>>> https://svn.boost.org/trac/boost/ticket/4189
>>> I ran Boost.Iterator regression tests with gcc 4.2 and it passed without
>>> error.
>>>
>>> Daniel Walker
>>
>> Maybe you should only conditionally wrap in a boost::optional, e.g., if
>> has_trivial_constructor<F>  is false
>
> What would that gain? It's a little hairy to implement on old
> compilers. I'm not sure that it's worth it.

The gain is you don't have to pay the cost of a boolean in space.  The 
boolean also has to be checked upon assignment and copy construction, so 
there is some amount of additional runtime overhead.

What is "it" in "it's a little hairy to implement on old compilers"? 
The has_trivial_constructor metafunction, or a 
has_trivial_constructor-aware transform_iterator?  For the former, I'd 
believe it, but I would expect the Boost.TypeTraits version to be good 
enough.  For the latter, (and I could be very very wrong here, but) 
seems like the only thing you have to do is put an indirection layer 
when calling the function held by the transform_iterator, e.g., 
unwrap_optional(f)( ...args... ).  In fact, there may be such a function 
provided by Boost.Optional...

- Jeff