Re: [Boost-users] Stack based "pimpl"

From: Michal Kandulski (Michal.Kandulski_at_[hidden])
Date: 2005-08-09 03:37:33

Douglas Gregor wrote:
> On Aug 5, 2005, at 2:07 PM, Michal Kandulski wrote:
> 
> 
>>Hi!
>>
>>I'd like to implement a pimpl, pointing to a stack "allocated" buffer
>>by means of palcement new/destroy.
>>
>>There would be no problem but there are alignment issues.
>>
>>What really I need is:
>>- a way to check by te means of Boost wheater that an object of class
>>   X can be placed (with placement new) in a buffer of given adress
>>   and size:
>>- a way of getting a pointer to X (within the buffer) where the
>>   object could be paced
> 
> 
> boost::optional<T> does this, as does the more-complicated 
> boost::variant.
> 
> You'll want to use the type traits alignment_of (to find the alignment 
> of a type) and type_with_alignment (to find a POD type with a given 
> alignment). If you dig around Boost for aligned_storage you'll see the 
> technique we've used.
> 
> 	Doug

Thanks for the answer, but I've already checked both optional and variant.

OK. I can get a POD type suitable to store an object of X, but
my problem is: how to determine whether the object can be stored in
a given stack "allocated" buffer of given size.

My input data is:
     - alignment_of<X>::value,
     - sizeof(X),
     - (aligned_storage<alignment_of<X>::value, sizeof(X)>::type)
     - max_size
     - m_impl_buffer[max_size]

And the question is again: can store an object of X in m_impl_buffer 
and if I can - where?

Michal