[variant][any] template parameterized variant
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? 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. to simplify the question: template<class T> class dummy{ ... }; variant< "dummy<T>" > v; //for all T If it simplifies things, suppose that all dummy<T> have the same sizeof and that all are convertible to each other. Or should I be using an extension of boost::any with some adhoc restriction over the possible types on construction? Thank you, Alfredo
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.
On Sat, Oct 9, 2010 at 5:52 AM, Mathias Gaunard <mathias.gaunard@ens-lyon.org> 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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participants (4)
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alfC -
Alfredo Correa
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Igor R -
Mathias Gaunard