Re: [Boost-users] variant over base and derived classes
-----Original Message----- From: Hicham Mouline [mailto:hicham@mouline.org] Sent: 09 December 2010 00:04 To: 'boost-users@lists.boost.org' Subject: variant over base and derived classes
hello, <snip>
typedef boost::variant<S,Deriv1, Deriv2, Deriv3> my_variant_t;
Deriv3 d3;
struct print_type: public boost::static_visitor<> { template <typename T> void operator()(const T&) const { std::cout<< typeid(T).name() <<std::endl; } };
const S& s = d3; my_variant_t v = s; boost::apply_visitor( print_type(), v );
This prints S.
Is it a truncated copy of d3 that is stored v? Yes. How do I construct v polymorphically so that the real object is stored?
regards, You have to downcast before creating the variant. boost::variant only uses the static type of the object. In Christ, Steven Watanabe
If I am given a const S* sptr, how do I construct an instance of the above variant then? const Deriv1* d1ptr; const Deriv2* d2ptr; const Deriv3* d3ptr; my_variant_t v = ((d1ptr = dynamic_cast<const Deriv1*>(sptr)) != 0 )? *d1ptr: ((d2ptr = dynamic_cast<const Deriv2*>(sptr)) != 0 )? *d2ptr: ((d3ptr = dynamic_cast<const Deriv3*>(sptr)) != 0 )? *d3ptr: *sptr; Is there a better way of writing this? rds, -------------------------------------------- Trying to do this more generically, I've thought of call mpl::for_each on the ::types of the variant to see which dynamic_cast works. The problem is that I need the most derived type to be stored in the variant. I can see how to do it by a virtual function but that is completely intrusive. regards,
(sorry if this is a top post, my phone shows no msg quotes) I'm no expert on this, but isn't there an compile time function to testing if an instance is derived from another (in introspection or something)? You could then maybe sort the variant types for most to least derived and then test untill you find the suitable cast? Just brainstorming pretty much. Hope its useful! Best, Dee On 09/12/2010, Hicham Mouline <hicham@mouline.org> wrote:
-----Original Message----- From: Hicham Mouline [mailto:hicham@mouline.org] Sent: 09 December 2010 00:04 To: 'boost-users@lists.boost.org' Subject: variant over base and derived classes
hello, <snip>
typedef boost::variant<S,Deriv1, Deriv2, Deriv3> my_variant_t;
Deriv3 d3;
struct print_type: public boost::static_visitor<> { template <typename T> void operator()(const T&) const { std::cout<< typeid(T).name() <<std::endl; } };
const S& s = d3; my_variant_t v = s; boost::apply_visitor( print_type(), v );
This prints S.
Is it a truncated copy of d3 that is stored v? Yes. How do I construct v polymorphically so that the real object is stored?
regards, You have to downcast before creating the variant. boost::variant only uses the static type of the object. In Christ, Steven Watanabe
If I am given a const S* sptr, how do I construct an instance of the above variant then?
const Deriv1* d1ptr; const Deriv2* d2ptr; const Deriv3* d3ptr; my_variant_t v = ((d1ptr = dynamic_cast<const Deriv1*>(sptr)) != 0 )? *d1ptr: ((d2ptr = dynamic_cast<const Deriv2*>(sptr)) != 0 )? *d2ptr: ((d3ptr = dynamic_cast<const Deriv3*>(sptr)) != 0 )? *d3ptr: *sptr;
Is there a better way of writing this?
rds,
--------------------------------------------
Trying to do this more generically, I've thought of call mpl::for_each on the ::types of the variant to see which dynamic_cast works. The problem is that I need the most derived type to be stored in the variant. I can see how to do it by a virtual function but that is completely intrusive.
regards,
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Hicham Mouline