为什么基类函数不被覆盖？

Question

I override the base-class function ShowProperties in a sub-class, but still the base-class function gets called. Why is this?

class hierarchy:

class CDiagramEntity : public CObject
{
public:
  virtual void ShowProperties( CWnd* parent, BOOL show = TRUE );
}

class CNetworkSymbol : public CDiagramEntity
{
  /*NO 'ShowProperties' Function*/
}

class CDeviceEntity : public CNetworkSymbol
{
  /*NO 'ShowProperties' Function*/
}

class CSwitch : public CDeviceEntity
{
public:
  virtual void ShowProperties( CWnd* parent, BOOL show = TRUE );
}

Use:

/*Use Here*/
{
  CDiagramEntity* obj = GetSelectedObject();
  if( obj )
  {
    CSwitch* sw = (CSwitch*)obj;
     sw->ShowProperties( this );    
     /*calls CDiagramEntity's function, not CSwitch's function*/
  }
}

PS:

1. As @iammilind suggests, I removed virtual from CDiagramEntity and use casting, the function of CSwitch is called, and from the properties it shows(the properties can only be changed to what it shows when CSwitch is created), I am quite sure it is a CSwitch.

2. As @user1610015 comments, if I use

    CSwitch* sw = dynamic_cast<CSwitch*>(obj); 

   it returns NULL .

3. As @Andrian Sham says, I got the problem's reason:

    GetSelectedObject()--->finally calls---> { CDiagramEntity* result = NULL; if ( index < m_objs.GetSize() && index >= 0 ) result = static_cast< CDiagramEntity* >( m_objs.GetAt( index ) ); /*m_objs is defined as: CObArray m_objs;*/ return result; } 

m_objs:

CObArray m_objs;

And the object is stored as:

...( CDiagramEntity* obj )
{
  obj->SetParent( this );
  m_objs.Add( obj );
  SetModified( TRUE );
}

But can someone explain this in more details?

Answer 1

I am not sure how you do your debug. You said you can see a member var of CSwitch. If you mean you inspect CSwitch* sw = (CSwitch*)obj; and see such var in sw , you are in fact getting in wrong. Debugger is simply "interpreting" the piece of memory pointed by sw as it is a CSwitch. It doesn't mean that it is really a CSwitch.

I can only thought of a case. Have you put the object instance in any STL container and get it out to put it as the select item?

(I forgot MFC totally, this is just an example)

vector<CDiagramEntity> entities;  // note it is of type CDiagramEntity, not CDiagramEntity*

CSwitch aCSwitch;
entities.push_back(aCSwitch);

// later
SetItem("SomeValue", entities[i]);

You may think you entities[i] should return an instance of CSwitch but it is not.

(This is the best I can guess if you are pretty sure that the selected item should be a CSwitch)

Answer 2

Because the ShowProperties() is a virtual function.

When virtual mechanism kicks in, the function binding happens at runtime .
If ShowProperties() is returning an object of class CDiagramEntity , then CDiagramEntity::ShowProperties() will be called irrespective of static type of type calling pointer (here sw ).

Just for experiment purpose, remove the virtual keyword and run the code. You will get the expected behavior. But beware of possible undefined behavior.

Answer 3

Because even if you cast a CDiagramEntity object to a CSwitch object, the call is made throgh virtual dispatch. So, if the object returned by GetSelectedObject is not actually a CSwitch, CDiagramEntity's ShowProperties is called.

Why are you casting the object to CSwitch? It doesn't make sense from a design perspective. You have a polymorphic object, and yet you want to treat it as one of its specialized derivations.