Re: [Boost-users] Recursive variant problem

Subject: Re: [Boost-users] Recursive variant problem
From: Timothy Madden (terminatorul_at_[hidden])
Date: 2010-07-08 11:26:20

Larry Evans wrote:
> On 07/08/10 02:11, Timothy Madden wrote:
>  > Larry Evans wrote:
> [snip]
>  >> Maybe you could just use pointers to the enclosing composite, like in:
>  >>
>  >> #include <tr1/tuple>
>  >> #include <vector>
>  >>
>  >> struct tuple_tree
>  >> : std::tr1::tuple
>  >>   < float
>  >>   , int
>  >>   , std::vector<tuple_tree*>
>  >>   >
>  >> {
>  >> };
>  >
>  > I can use vector<tuple_tree> directly, with no pointer needed, and it
>  > works,
> 
> Hmm...  I'm surprised.  I would have thought a pointer would be
> needed to break the recursion because, otherwise, the tuple_tree CTOR
> would create an infinite tree.  But now that I think more about it,
> the pointer is within the std::vector.  No infinite tree is created
> because the vector is initially empty.

Well, as you can see, some people say you were correct, after all, 
because C++ might place a requirement to always use complete types for 
containers.

> 
>  > but I would expect boost::make_recursive_variant<> to do the
>  > substitution correctly so I could create the type in-place.
>  >
>  > This structure is part of a bigger tuple of containers in my code, and
>  > using tuple_tree would require me to declare it in advance, separately,
>  > while make_recursive_variant would create the needed type in-place.
>  >
>  > Here is my code:
[...]
>  >     std::vector
>  >       <
>  >         boost::tuple
>  >           <
>  >             std::basic_string<TCHAR>,   // uuid
>  >             int,                      // revision
>  >             std::basic_string<TCHAR>,   // name
>  >             std::basic_string<TCHAR>,   // desc
>  >             std::basic_string<TCHAR>,   // status
>  >             std::basic_string<TCHAR>,   // statusName,
>  >             std::basic_string<TCHAR>,   // memo
>  >             std::basic_string<TCHAR>,   // designFileName
>  >             std::vector
>  >               <
>  >                 UIOVarEntry
> 
> 
> What happens if you just use this,  IOW, just
> std::vector<UIOVarEntry>.  There'd be no infinite tree created
> for the reasons cited earlier.  Wouldn't this satisfy your
> requirements?
[...]

It bothers me that to compose my large structure I need some previously 
declared class UIOVarEntry, to hold a certain number of components from 
my big typedef. With make_recursive_variant I could have all my 
structure declared in-place at once.

Thank you,
Timothy Madden