Re: [Boost-users] Re: Can you use a template class to store data in a vertex of a boost graph?

From: Cory Nelson (phrosty_at_[hidden])
Date: 2004-11-14 21:08:43

boost.variant :)

On Sun, 14 Nov 2004 17:54:51 -0500, Jeffrey Holle
<jeff.holle_at_[hidden]> wrote:
> Something like:
> 
> class Data
> {
>   Data(float item) : type(float),float_datum(item) {}
>   Data(uint32_t item) : type(uint32_t),uint32_datum(item) {}
> private:
>   enum { float, uint32_t} type;
>   union {
>     float float_datum;
>     uint32_t uint32_datum;
>   };
> }
> 
> This is called an anonymous union.
> The caviot is C++ does allow classes within unions.  Its because the
> compiler wouldn't know which default constructor to call...
> If you need to handle classes, its time to start using pointers too.
> 
> 
> 
> Stephen torri wrote:
> > On Sun, 2004-11-14 at 10:42 -0500, Jeffrey Holle wrote:
> >
> >>You can't use templates in this manner.
> >>Ask yourself, which type of Data object do you expect boost.graph to
> >>instanciate when a new vertex is created?  It can only make one choice.
> >
> >
> > I know that this was broken. I made the declaration that it was.
> >
> >
> >>If I was doing this, and was concerned about memory consumption, I'd
> >>consider using a union attribute in Data, along with a type attribute.
> >
> >
> > Can you provide a simple example using the two types I gave in this
> > example: float and uint32_t?
> >
> > Stephen
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-- 
Cory Nelson
http://www.int64.org