Boost logo

Boost Users :

Subject: Re: [Boost-users] variant over base and derived classes
From: Hicham Mouline (hicham_at_[hidden])
Date: 2010-12-09 00:59:03


>> -----Original Message-----
>> From: Hicham Mouline [mailto:hicham_at_[hidden]]
>> Sent: 09 December 2010 00:04
>> To: 'boost-users_at_[hidden]'
>> Subject: variant over base and derived classes
>>
>> hello,
>>
>> How to determine the actual type of the object stored in a variant?
>>
>> struct S {
>> virtual ~S()
>> {}
>> };
>>
>> struct Deriv1 : public S {
>> virtual ~Deriv1()
>> {}
>> };
>>
>> struct Deriv2 : public S {
>> virtual ~Deriv2()
>> {}
>> };
>>
>> struct Deriv3 : public Deriv2 {
>> virtual ~Deriv3()
>> {}
>> };
>>
>> 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,


Boost-users list run by williamkempf at hotmail.com, kalb at libertysoft.com, bjorn.karlsson at readsoft.com, gregod at cs.rpi.edu, wekempf at cox.net