Re: [Boost-users] Determining Type and Extracting Data from private class member vector<boos::variant<> >

10 Jan 2005

      On Fri, 7 Jan 2005 14:07:14 -0600, Matthew Jankowski
<mattjankowski@yahoo.com> wrote:
...
Hi all, I'm new to Boost and have some questions about how to get data out
of a vector<boost::variant<> > container. For this purpose I put together
'class Base' which holds a vector<boost::variant <Base*, DerivedBase*> >
Pointers. I wrote a function template to insert pointers, but I have real
difficulty extracting pointers.
I'm not sure what you're trying to do makes much sense to do with
Boost.Variant.  It is more suited to storing objects than pointers.

Why not just a std::vector<boost::shared_ptr<Base> >>?  I'm assuming
your DerivedBase object inherits from Base and you're trying to avoid
slicing by storing pointers in your container.

If you're insistent on using Variant, you can use get<T> to pull out a
value by its type:

#include <vector> 
#include <iostream> 
#include <algorithm> 

#include <boost/variant.hpp> 

using namespace std; 
using namespace boost; 

struct base {}; 
struct derived : public base {}; 

typedef variant<base*, derived*> holder; 

struct is_base { 
    bool operator () (holder& h) const { 
        return get<base*> (&h) != 0; 
    } 
}; 

struct is_derived { 
    bool operator () (holder& h) const { 
        return get<derived*> (&h) != 0; 
    } 
}; 

int 
main () 
{ 
    vector<holder> geoffrey; 
    geoffrey.push_back (new base); 
    geoffrey.push_back (new base); 
    geoffrey.push_back (new base); 
    geoffrey.push_back (new derived); 
    geoffrey.push_back (new derived); 

    cout << "base: " << count_if (geoffrey.begin (), geoffrey.end (), 
                                  is_base ()) 
         << ", derived: " << count_if (geoffrey.begin (), geoffrey.end (), 
                                       is_derived ()) 
         << endl; 
} 

-- 
Caleb Epstein
caleb dot epstein at gmail dot com