Re: [boost] Uninitialized storage container to delay construction?

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2007-06-03 11:40:01

----- Mensaje original -----
De: "Aaron W. LaFramboise" <aaronrabiddog51_at_[hidden]>
Fecha: Domingo, Junio 3, 2007 5:12 pm
Asunto: [boost] Uninitialized storage container to delay construction?
Para: boost_at_[hidden]

> Does Boost have some sort of uninitialized storage container? 
> Presently, I use this pattern if I want to delay construction of a 
> sub-object.
> 
> class my_class {
>   public:
>     my_class() {
>       // ...
>       subobject = auto_ptr<type>(new type(params));
>     }
>   private:
>     auto_ptr<type> subobject;
> };
> 
> But I'd like something similar that didn't use the heap, but 
> instead 
> allocated the storage for the sub-object within the class object 
> itself. 
>  Perhaps it would look like this.
> 
> class my_class2 {
>   public:
>     my_class2() {
>       // ...
>       subobject.construct(params);
>     }
>   private:
>     uninitialized_container<type> subobject;
> };
> 
> A TR1 array<T, sizeof(T)> almost works, except for these points:
>   (1) It doesn't provide proper alignment guarantees.
>   (2) It doesn't call the destructor when it is destroyed.
> 
> So, does Boost have any such thing?

I think you want to use aligned_storage and alignment_of
from Boost.TypeTraits:

  aligned_storage<
    sizeof(type),alignment_of<type>::value>::type subobject;

which provides the alignment guarantees. (2) OTOH is not taken
care of by this construct, so it' still your responsibility
to clean up subobject (by doing something like
((type*)&subobject)->~type()) when appropriate.

HTH,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo