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From: Allen Bierbaum (allenb_at_[hidden])
Date: 2003-04-04 17:10:27

Eric Friedman wrote:
> Allen Bierbaum wrote:
>>I have been very impressed with the Variant library and started using it
>>with Boost 1.29.
> Good to hear. I'd be interested in your experience using the library in a
> real-world (?) application.

I was able to get variant working with 1.30 by pulling some of the code
out of the sandbox.

I have started using it in my application and have run into one major
limitation: variant size (in number of allowed types)

I need to use a variant of about 15-20 allowed types. By default
variant only works with 10 types. I know that by defining:


I can get the limit up to 20 or 30 or whatever I want. Unfortunately
this also ups the limits on *all* lists that may be in the code and
makes compile times take much longer since I can't use the preprocessed

Anyway, looking back through the list I found a reference here:

that says the recommended way to do this type of thing when users want
longer list types is to use the numbered list types. I know this
doesn't directly relate to variant, but I was thinking it would be very
nice to have something similar for variant.

So for example I could say:


to get a variant that allows 27 types.

I would also be happy with something like:

boost::variant<27, type1, type2,..., type27>

but I could see where some people would not like this form since it
requires the size to be explicitly set.

Maybe something like this would work better.

boost::variantS<27, type1, ..., type27>

This provides an explicit type that is used when you want the first
parameter to be the size, but still allows the normal boost::variant
type to automatically detect the size.

What do you think, are either of these two options feasible (or is there
already support for it that I missed)?


>>I now need to start to using Boost 1.30.0 though and I don't think the
>>variant library distributed for the review will work with the 1.30.0
>>Does anyone have any suggestions how I can use the variant library with
>>Boost 1.30.0? Anyone have an updated distribution of the Variant library?
> I plan to ready the code for release with 1.31. Until then, you can try
> using the code in the sandbox, which I have recently updated to work with
> 1.30.
> Note, however, that the code in the sandbox will change -- sometimes
> significantly -- and integrates variant with other libraries I am
> developing, such as Boost.Visitor and Boost.Move. If this is a problem for
> you, you may want to update the variant submission code yourself. Most of
> the required changes are to accomodate class and header name-changes in MPL
> between 1.29 and 1.30 -- nothing terribly significant.
> Thanks,
> Eric
> _______________________________________________
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-- Allen Bierbaum                          allenb_at_[hidden] --
-- VR Juggler Team                      --
-- Virtual Reality Applications Center --

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