Re: [Boost-users] Recursive variant problem

Subject: Re: [Boost-users] Recursive variant problem
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-07-08 08:35:59

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.

 > 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:
 >
 > class UIOVarEntry:
 > public
 > boost::tuple
 >   <
 >     std::basic_string<TCHAR>,         // id
 >     std::basic_string<TCHAR>,         // name
 >     std::basic_string<TCHAR>,         // desc
 >     std::basic_string<TCHAR>,         // type
 >     std::basic_string<TCHAR>,         // value
 >     std::vector<UIOVarEntry>          // childList
 >   >
 > {
 > };
 >
 > typedef
 > boost::tuple
 >   <
 >     int,             // serviceID
 >     int,             // projectID
 >     int,             // version
 >     std::basic_string<TCHAR>,    // htmlData,
 >     std::basic_string<TCHAR>,    // uioDesignData
 >     std::vector
 >       <
 >         boost::tuple
 >           <
 >             bool,                     // isGroup
 >             std::basic_string<TCHAR>,   // uuid
 >             int,                      // version
 >             std::basic_string<TCHAR>,   // parentID
 >             std::basic_string<TCHAR>,   // prevID
 >             std::basic_string<TCHAR>,   // name,
 >             std::basic_string<TCHAR>,   // desc
 >             std::basic_string<TCHAR>,   // filename
 >             std::vector
 >               <
 >                 boost::tuple
 >                   <
 >                     std::basic_string<TCHAR>,   // slotID,
 >                     int,                      // revision
 >                     std::basic_string<TCHAR>,   // uioID
 >                     std::basic_string<TCHAR>,   // layout
 >                     std::basic_string<TCHAR>,   // variables
 >                     std::basic_string<TCHAR>    // imgName
 >                   >
 >               >                 // slotList
 >           >
 >       >,            // planPageList
 >     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?

 >             /*
 >                 boost::make_recursive_variant
 >                   <
 >                     boost::tuple
 >                       <
 >                         std::basic_string<TCHAR>,         // id
 >                         std::basic_string<TCHAR>,         // name
 >                         std::basic_string<TCHAR>,         // desc
 >                         std::basic_string<TCHAR>,         // type
 >                         std::basic_string<TCHAR>,         // value
 >                         std::vector<boost::recursive_variant_>    //
 > childList
 >                       >,
 >                     int,
 >                     float
 >                   >::type
 >             */
 >               >,              // varList
 >             std::vector< std::basic_string<TCHAR> > // wireframeList
 >           >
 >       >         // uioList
 >   >
 >     ProjectData;
 >
 >
 > Thank you,
 > Timothy Madden