fork download

copy

 
#include <cassert>
#include <iostream>
#include <memory>
 
using namespace std;
 
 
enum class ObjectCategory
	{
	none,
	XObject,
	YObject,
	XYObject
	};
 
 
class ObjectBase:
	public enable_shared_from_this< ObjectBase >
	{
	public:
 
		ObjectBase() {}
 
		virtual ~ObjectBase() {}
 
		virtual ObjectCategory Category() const = 0;
 
	};
 
class XObject:
	public virtual ObjectBase
	{
	public:
 
		XObject() {}
 
		~XObject() {}
 
		ObjectCategory Category() const override
			{
			return ObjectCategory::XObject;
			}
 
	};
 
int main()
	{
	typedef shared_ptr< ObjectBase > ObjectPtr;
 
	ObjectPtr p1 = make_shared< XObject >();
	assert( ObjectCategory::XObject == p1->Category() );
 
	auto x1 = static_pointer_cast< XObject, ObjectBase >( p1 );
	assert( x1 );
 
	return EXIT_SUCCESS;
	}
 

In file included from /usr/include/c++/6/memory:82:0,
                 from prog.cpp:4:
/usr/include/c++/6/bits/shared_ptr.h: In instantiation of ‘std::shared_ptr<_Tp1> std::static_pointer_cast(const std::shared_ptr<_Tp2>&) [with _Tp = XObject; _Tp1 = ObjectBase]’:
prog.cpp:54:59:   required from here
/usr/include/c++/6/bits/shared_ptr.h:447:35: error: cannot convert from pointer to base class ‘ObjectBase’ to pointer to derived class ‘XObject’ because the base is virtual
     { return shared_ptr<_Tp>(__r, static_cast<_Tp*>(__r.get())); }
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Standard output is empty