About downcasting from base class to subclass pointer

Question

A static check tool shows a violation on the below code:

class CSplitFrame : public CFrameWnd  
...
class CVsApp : public CWinApp
CWnd* CVsApp::GetSheetView(LPCSTR WindowText)
{
 CWnd* pWnd = reinterpret_cast<CSplitFrame*>(m_pMainWnd)->m_OutputBar.GetChildWnd(WindowText);
 return pWnd;
}

Error Message: Class 'CSplitFrame' inherits from class 'CWnd'

Description: Avoid casts down the inheritance hierarchy. This rule detects casts from a base class pointer to a subclass pointer.

Benefits: Allowing casts down the inheritance hierarchy leads to maintenance problems, and downcasting from a base class is always illegal.

References:

1. Scott Meyers, "Effective C++: 50 Specific Ways to Improve Your Programs and Design", Second Edition, Addison-Wesley, (C) 2005 Pearson Education, Inc., Chapter: "Inheritance and Object-Oriented Design", Item 39
2. JOINT STRIKE FIGHTER, AIR VEHICLE, C++ CODING STANDARDS Chapter 4.23 Type Conversions, AV Rule 178

Do you think it's a good practice for not casting down from a base class pointer to a subclass pointer? Why and When should I follow this rule?

Answer 1

reinterpret_cast is certainly a bad idea here, regardless of coding standards or OOP theory. It has to be dynamic_cast or boost::polymorphic_downcast.

As for the chapter 39 of Effective C++, it concentrates on the maintenance problems caused by having to downcast to multiple different types and having to check the return values of dynamic_cast for potential failures, resulting in multiple branches in the code:

The important thing is this: the if-then-else style of programming that downcasting invariably leads to is vastly inferior to the use of virtual functions, and you should reserve it for situations in which you truly have no alternative.

Answer 2

Let us go through some of the downcasting example in MFC:

CButton* from CWnd*

CWnd* wnd = GetDlgItem(IDC_BUTTON_ID);
CButton* btn = dynamic_cast<CButton*>(wnd);

CChildWnd* from CFrameWnd*

CChildWnd * pChild = ((CSplitFrame*)(AfxGetApp()->m_pMainWnd))->GetActive();

There are indeed some of the limitation of MFC design.

Due to CWnd provides the base functionality of all window classes in MFC, it does even serve as a base class of View, Dialog, Button etc.

If we want to avoid downcasting, probably we need MFC hacker to split CWnd into fewer pieces?

Now, comes to the another question, how to solve the violation, my humble opinion is try to avoid unsafe downcasting, by using safe downcasting:

Parent *pParent = new Parent;
Parent *pChild = new Child;

Child *p1 = static_cast<Child*>(pParent);   // Unsafe downcasting:it assigns the address of a base-class object (Parent) to a derived class (Child) pointer
Parent *p2 = static_cast<Child*>(pChild);   // Safe downcasting:it assigns the address of a derived-class object to a base-class pointer

It serve as good practise for using safe downcasting, even though the violation is still exists, we will just suppress the violation with given explanation.

Few of the useful reference:
http://support.microsoft.com/kb/108587
http://blog.csdn.net/ecai/archive/2004/06/26/27458.aspx
http://www.codeproject.com/KB/mcpp/castingbasics.aspx
http://www.bogotobogo.com/cplusplus/upcasting_downcasting.html

Lastly, thanks for various useful response from all of you.
They are indeed very helpful.