Why is DECLARE_DYNAMIC & IMPLEMENT_DYNAMIC nessary for DYNAMIC_DOWNCAST? - c++

Question

I have two classes:
/*Switch.h*/
class CSwitch : public CDeviceEntity {}
/*EndSystem.h*/
class CEndSystem : public CDeviceEntity {}
but when I use:
CDeviceEntity* dev = NULL;
dev = topo->headList[i]->node;
if ( DYNAMIC_DOWNCAST( CEndSystem, dev ) != NULL ) {}
"DYNAMIC_DOWNCAST" always returns not NULL while dev is kind of class CEndSystem or class CSwitch.
If use:
/*Switch.h*/
class CSwitch : public CDeviceEntity { DECLARE_DYNAMIC(CSwitch) }
and
/*Switch.cpp*/
IMPLEMENT_DYNAMIC(CSwitch, CDeviceEntity)
/*EndSystem.h*/
class CEndSystem : public CDeviceEntity { DECLARE_DYNAMIC(CEndSystem) }
and
/*EndSystem.cpp*/
IMPLEMENT_DYNAMIC(CEndSystem, CDeviceEntity)
"DYNAMIC_DOWNCAST" returns NULL or not NULL according to class CEndSystem or class CSwitch.
Why "DECLARE_DYNAMIC" and "IMPLEMENT_DYNAMIC" are nessary for "DYNAMIC_DOWNCAST"?
/*Algorithm.h*/
static int getESNum();
/*Algorithm.cpp*/
int CAlgorithm::getESNum()
{
int count = 0;
CDeviceEntity* dev = NULL;
for (int i = 0; i < topo->nodeNum; i++)
{
dev = topo->headList[i]->node;
if ( DYNAMIC_DOWNCAST( CEndSystem, dev ) != NULL )
{
count++;
}
}
return count;
}
/*Algorithm.h*/
static int getSWNum();
/*Algorithm.cpp*/
int CAlgorithm::getSWNum()
{
int count = 0;
CDeviceEntity* dev = NULL;
for (int i = 0; i < topo->nodeNum; i++)
{
dev = topo->headList[i]->node;
if ( DYNAMIC_DOWNCAST(CSwitch, dev) != NULL )
{
count++;
}
}
return count;
}
And the functions are called in serialization when saving the document.

Answer 1

DYNAMIC_DOWNCAST is a throw back to how you used to have to do dynamic casting before RTTI information was available from the compiler. The casting information is created using the macros DECLARE_DYNAMIC and IMPLEMENT_DYNAMIC which use the class CRuntimeClass to decide if the cast is valid.
DYNAMIC_DOWNCAST simply does this:
CObject* AFX_CDECL AfxDynamicDownCast(CRuntimeClass* pClass, CObject* pObject)
{
if (pObject != NULL && pObject->IsKindOf(pClass))
return pObject;
else
return NULL;
}
The DECLARE_DYNAMIC macro adds this code:
#define DECLARE_DYNAMIC(class_name) \
protected: \
static CRuntimeClass* PASCAL _GetBaseClass(); \
public: \
static const CRuntimeClass class##class_name; \
static CRuntimeClass* PASCAL GetThisClass(); \
virtual CRuntimeClass* GetRuntimeClass() const; \
Add IMPLEMENT_DYNAMIC adds this code:
#define IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, wSchema, pfnNew, class_init) \
CRuntimeClass* PASCAL class_name::_GetBaseClass() \
{ return RUNTIME_CLASS(base_class_name); } \
AFX_COMDAT const CRuntimeClass class_name::class##class_name = { \
#class_name, sizeof(class class_name), wSchema, pfnNew, \
&class_name::_GetBaseClass, NULL, class_init }; \
CRuntimeClass* PASCAL class_name::GetThisClass() \
{ return _RUNTIME_CLASS(class_name); } \
CRuntimeClass* class_name::GetRuntimeClass() const \
{ return _RUNTIME_CLASS(class_name); }
#define IMPLEMENT_DYNAMIC(class_name, base_class_name) \
IMPLEMENT_RUNTIMECLASS(class_name, base_class_name, 0xFFFF, NULL, NULL)
I imagine few people still use this for new projects, instead preferring the C++ standard dynamic_cast<> call (along with static_cast and reinterpret_cast).