c++ - SmartPointer : cast between base and derived classes -

say have function :

smartptr<a> dosomething(smartptr<a> a);

and classes :

class { } class b : public { }

and :

smartptr<a> foo = new b(); dosomething(foo);

now, smartptr<b> object dosomething.

smartptr<b> b = dosomething(foo);

is possible ? kind of casting have ?
right now, found believe ugly :

b* b = (b*)dosomething().get()

important notes : not have access smartptr , dosomething() code.

instead of doing that, can :

b *b = dynamic_cast< b* >( dosomething.get() );

but have check if b null.

Score