On Sat, Oct 9, 2010 at 5:52 AM, Mathias Gaunard 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, 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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