Re: [Boost-users] type_with_alignment

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2006-06-14 11:32:31

On Jun 14, 2006, at 10:22 AM, David Abrahams wrote:

> Howard Hinnant <hinnant_at_[hidden]> writes:
>
>> There are applications such as boost::function where aligned storage
>> for an unknown type is useful.  In boost::function's case, aligned
>> storage could be used to implement the "short string optimization".
>
> And so it is, AFAIK.
>
>> The general pattern is:
>>
>> class Container
>> {
>> private:
>>      const unsigned buf_size = ...;
>>      aligned_storage<buf_size>::type buffer;
>> public:
>>      template <class T>
>>         Container(const T& t) {/* might stick t into buffer instead
>> of on heap, based on sizeof(T) */}
>> };
>>
>> On Jun 14, 2006, at 6:55 AM, David Abrahams wrote:
>>
>>> I think what the OP is looking for is called boost::shared_ptr ;-)
>>
>> shared_ptr can't compete with this application performance-wise.
>
> Well, it depends how expensive it is to copy your small object.  If it
> turns out that T is a vector, and it fits in your local buffer, when
> Container is copied it might be a lot cheaper to use shared_ptr.

Sure.  I'm responding to the assertion that you need a known complete  
type to effectively use aligned_storage.  My point is that there  
exist valuable use cases where this assertion is false.  The  
existence of these cases is what makes the defaulted alignment  
template parameter on aligned_storage valuable.

Tony's observation is that this default value is even more valuable  
if it is dependent upon the buffer size, which again I fully agree  
with.  If you want a 5 byte buffer that is aligned to the "most  
stringent alignment requirements" (definition found in 5.3.4p10), an  
alignment of 16 (for example) is likely to be correct but overkill.   
I had personally settled on 8 for this particular example but Tony  
makes a convincing argument for 4 being the optimum (all numbers  
assuming current popular desktop processors and quoted just for  
example).

-Howard