On Tue, Dec 23, 2008 at 4:54 AM, Jason Cipriani <jason.cipriani@gmail.com> wrote:

Research is showing this isn't possible. I'm sure there's a good reason for it... but I need to get this working. I came up with this hack that works but is scary. Assuming there is no virtual inheritance involve (this would have the same semantics as reinterpret_cast):

template <typename Dest, typename Src> shared_ptr<Dest> shared_reinterpret_cast (const shared_ptr<Src> &src) {

shared_ptr<Src> hack(src); shared_ptr<Dest> dest;

assert(sizeof(hack) == sizeof(dest)); char buf[sizeof(dest)];

// swap dest and hack memcpy(buf, &dest, sizeof(dest)); memcpy(&dest, &hack, sizeof(dest)); memcpy(&hack, buf, sizeof(hack));

return dest;

}

Scratch that, this seems safer: template <typename Dest, typename Src> shared_ptr<Dest> shared_reinterpret_cast (const shared_ptr<Src> &src) { shared_ptr<Src> hack(src); return *(shared_ptr<Dest> *)&hack; } The first attempt would cause problems if shared_ptr's implementation stored a pointer to itself, for some reason, or gave it's address to some other shared_ptr. Jason

It's really dirty, but will it work OK? The basic idea is to swap a new shared_ptr<Base> with an empty shared_ptr<Derived>, but bypass shared_ptr::swap()'s type checking.

Jason

On Tue, Dec 23, 2008 at 4:36 AM, Jason Cipriani <jason.cipriani@gmail.com> wrote:

...

If I have:

class Base { }; class Derived : public Base { };

And a container, e.g.:

std::vector<boost::shared_ptr<Base> > objs;

How do I downcast to a shared_ptr<Derived>? E.g.:

boost::shared_ptr<Derived> d = (boost::shared_ptr<Derived>)objs[0];

I have a function that takes a shared_ptr<Derived> and all I have is a shared_ptr<Base>. I don't mind static_cast behavior, although dynamic_cast behavior would be ideal.

Thanks, Jason

2008/12/23 Vincent Agnus <vincent.agnus@ircad.fr>:

Igor R wrote:

...

2008/12/23 Jason Cipriani <jason.cipriani@gmail.com>:

...

If I have:

class Base { }; class Derived : public Base { };

And a container, e.g.:

std::vector<boost::shared_ptr<Base> > objs;

How do I downcast to a shared_ptr<Derived>? E.g.:

boost::shared_ptr<Derived> d = (boost::shared_ptr<Derived>)objs[0];

Look at boost::static_pointer_cast

using class polymorphism boost::dynamic_pointer_cast is better.

Thanks, this is exactly what I wanted. Jason

You can also iterate on our vector as if the elements are of type Derivated with a code like this :

typedef std::vector<boost::shared_ptr<Base> > VectorBase;

template < class SubClass> struct DownCaster : public std::unary_function< boost::shared_ptr< Base >, boost::shared_ptr< SubClass> > { boost::shared_ptr< SubClass> operator()( boost::shared_ptr< Base > obj ) const { return boost::dynamic_pointer_cast< SubClass >( obj ); } };

typedef typename boost::transform_iterator< DownCaster<Derived> , VectorBase::iterator > CastIterator;

CastIterator start( objs.begin() );

boost::shared_ptr<Derived> d = *start; // is OK

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