Let's say I have two classes, A and B, where B is a child class of A.
I also have the following function:
void foo(boost::shared_ptr<const A> a)
{
boost::shared_ptr<const B> b = boost::dynamic_pointer_cast<const B>(a); // Error !
}
Compilation with gcc gives me the following errors:
C:\Boost\include/boost/smart_ptr/shared_ptr.hpp: In constructor 'boost::shared_ptr< <template-parameter-1-1> >::shared_ptr(const boost::shared_ptr<Y>&, boost::detail::dynamic_cast_tag) [with Y = const A, T = const B]':
C:\Boost\include/boost/smart_ptr/shared_ptr.hpp:522: instantiated from 'boost::shared_ptr<X> boost::dynamic_pointer_cast(const boost::shared_ptr<U>&) [with T = const B, U = const A]'
src\a.cpp:10: instantiated from here
C:\Boost\include/boost/smart_ptr/shared_ptr.hpp:259: error: cannot dynamic_cast 'r->boost::shared_ptr<const A>::px' (of type 'const class A* const') to type 'const class B*' (source type is not polymorphic)
What could possibly be wrong ?
Thank you.
Actually, I found out how to avoid this, but I'm not sure to understand.
My A
class was empty (and thus had no virtual destructor). If I add a virtual destructor, the error goes out. But I don't get it, why is this required ?
dynamic_pointer_cast
uses the C++ dynamic_cast
internally and dynamic_cast
requires your classes to have at least one virtual method. No virtual methods means no vtable and without vtable dynamic_cast
would not be able to figure which casts are doable at run-time.
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