Re: [boost] [fusion] fusion sequence of non-copy-constructible types

From: Joel de Guzman (joel_at_[hidden])
Date: 2008-07-07 09:22:47

Oliver Mueller wrote:
> hello,
> at the last boostcon i had the chance to be introduced to fusion by
> dan and joel. i appreciate the idea of fusion a lot and the library
> amazingly detailed. since we have already had great exposure to the
> mpl the fusion concepts present ample opportunity for new ideas.
> still, some little things sometimes make it hard for me to keep
> rolling with fusion:
> 
> i need to have a vector/list of non-copy-constructible types. using
> e.g. a standard fusion::vector that does not seem to be possible since
> objects are constructed first, then copied. of course, i could provide
> a public copy-constructor but that would in other places remove the
> compiler enforced contract that those objects are not copied under any
> circumstances.
> i also cannot think of a good reason why it is nesseccary to copy in that case.
> so is there a way of having a fusion sequence that does work on
> non-copy-constructible types?

Hmm... well...
a) How do you put a noncopyable object in a container?
b) How do you copy a container with noncopyable elements?

Fusion vector is based on stl vector. Even stl vector requires copy
constructible types.

One possibility is to store your object in a smart pointer or
wrap it in a reference wrapper. Then, the smart pointer and
reference wrapper can be stored in the fusion container.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net