[Boost-testing] boost::function Small Buffer Optimization (SBO) and allocators

Subject: [Boost-testing] boost::function Small Buffer Optimization (SBO) and allocators
From: Chris Hite (C.Hite_at_[hidden])
Date: 2009-12-03 12:56:43

I would really like to specify the SBO size on boost::function!  How 
about changing the signature to?:
template<typename Signature,std::size_t SBOSize=3*sizeof(void*)>
class function;

I've been trying to do a work around using the constructor with the 
allocator.  I was going to define my own function class derived from the 
boost one, with a private one time allocator using a private buffer.  
This approach works find as long as constructors where the Functor type 
is present.

The copy constructor and equals operators require serious hackery, since 
these constructors on boost function don't allow you to pass a new allow 
you to pass a new allocator.  There's no
  template<typename Allocator>
  functionN(const functionN&, Allocator);

This means that the Function constructor has to affect data underlying 
the const Function instance it's copying such that when the allocator is 
copied over it mutates into one that points at the private buffer of the 
new constructing instance.

This is doable, but kind of scary since I'm making assumptions about 
when and how the Allocator is copied.

This is also not such a wonderful solution, because the SBO is wasted.

Any chance of just parameterizing the SBO size?

Chris