[cfe-dev] __vector_base::__destruct_at_end

[Howard Hinnant]

On Jun 26, 2013, at 11:15 AM, Shriramana Sharma <samjnaa at gmail.com> wrote:

> Hi and thank you very much for your reply.
> 
> On Wed, Jun 26, 2013 at 8:29 PM, Howard Hinnant <hhinnant at apple.com> wrote:
>>> void __destruct_at_end(const_pointer __new_last, true_type) _NOEXCEPT;
>>> __destruct_at_end(const_pointer __new_last, false_type)
>> 
>> This is an optimization that I found was not working, so I disabled it with the intent of pulling it out completely if the disabilization went well.  It has, I'll pull it out.
> 
> So which of the two will remain? Do I presume the false_type version?
> (And I presume the true_type/false_type thing is an idiom to avoid
> creating a differently named delegate function? But a differently
> named delegate may clarify the purpose of the segregation, no?)

The false_type version will remain, folded into the __destruct_at_end(pointer __new_last).  This technique is used to chose among multiple implementations at compile time, without instantiating the unchosen branches.

> 
>>> Also: why separate all the containers into __container and
>>> __container_base with the former inheriting from the latter? Can't it
>>> be done in a single class?
>> 
>> This is a C++03 technique for making container constructors exception safe.  Let's look at an example:
> 
> Wow very nice explanation, thank you very much! I much prefer the
> try-catch model. Why isn't it then preferred in the libc++
> implemented? IMO less convoluted, and would avoid lots of base::method
> calls, no?

In the base class design the clean up code goes in one place:  ~base().  In the try-catch design the clean up code gets repeated in each constructor.  One could shuffle the clean up code off to a single function, but one still has to replicate the try/catch(...) {clean_up();} for each constructor.

Howard