[boost] status of variant type implementations & recursive variant types

From: Vesa Karvonen (vesa_karvonen_at_[hidden])
Date: 2002-03-26 04:26:23

There have been many discussions about variant types / discriminated unions 
on Boost. At the moment I'm working on something where I could use an 
efficient, recursive variant type. Before I consider implementing a variant 
type myself, I'd like to know what is the status of current variant type 
implementations?

Type safety is the whole point. I can statically specify all the types that 
the variant needs to be able to store. This means that Boost.Any has more 
flexibility than I need.

Efficiency is important. I simply can not afford unnecessary memory 
allocations. (This basically rules out Boost.Any.)

I also need to be able to build recursive variant types. Consider this Ocaml 
type definition:

  type value =
    | Int    of int
    | Float  of float
    | String of string
    | Aggr   of value array
                ^^^^^
Most of the variant type proposals I have seen do not support this.

I came up with the following syntax:

  struct value : variant<value // B&N (probably needed)
    , int
    , float
    , string
    , aggr<value>
    >
  {
    // ...
  };

(This kind of recursion does not work if the data structure (in this case 
aggr<>) wants to instantiate the type.)

This would, of course, require some constructors, etc... to be written by 
the user (those could be macroized).  Another technique would be to use some 
kind of placeholders, but those seem to require special support for each 
data structure (in this case aggr<>), which is something I do not like.

  typedef variant<int, float, string, aggr< variant_self > > value;

Does someone have an idea on how to support recursive variant types?

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