Re: [Boost-users] [variant][any] template parameterized variant

Subject: Re: [Boost-users] [variant][any] template parameterized variant
From: Alfredo Correa (alfredo.correa_at_[hidden])
Date: 2010-10-09 15:23:01

On Sat, Oct 9, 2010 at 5:52 AM, Mathias Gaunard
<mathias.gaunard_at_[hidden]> wrote:
> On 08/10/2010 20:42, alfC wrote:
>>
>> Hi,
>>
>>  I need a variant type object that can hold any type characterized by
>> a template. For example Boost.Units have a family of types
>> parameterized by "System".
>>
>>  quantity<  unit<dimension, System>, double>
>>
>> suppose dimension is fixed, e.g. typedef boost::units::length
>> dimension
>>
>> is it possible to define a variant over that family of types?
>
> No.
>
>> I could
>> create a make_variant_over<  list of all types>  but that can be that
>> list will be fixed and each time I defined a new System the list will
>> have to grow.
>
> Variant can only work with a fixed list of possible types.
>
>
>> Or should I be using an extension of boost::any with some adhoc
>> restriction over the possible types on construction?
>
> You could use type erasure.

ok, thanks for the tip. It seem that something like boost::any is the way to go.
It will be a special type of the type erasure because holded type always has the
the same sizeof. Therefore, ideally, it is a type erasure with no
necessity of dynamic allocation.

Does anybody knows a good reference or some Boost facility to
implement type erasure to start with?

I found this:
http://www.codeproject.com/KB/cpp/dynamic_typing.aspx
although it eliminates some dynamic allocation (by recycling memory
when possible) it doesn't remove it completely.

Thank you,
Alfredo

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