KB192099 - FIX: STATIC_DOWNCAST and DYNAMIC

Additional query words: DECLARE_DYNAMIC IMPLEMENT_DYNAMIC DECLARE_DYNCREATE IMPLEMENT_DYNCREATE DECLARE_SERIAL IMPLEMENT_SERIAL IsKindOf

SYMPTOMS

STATIC_DOWNCAST( class_name, pobject )

   class_name

The name of the class you want to cast to.

 
   pobject

The pointer to be cast to a pointer to a class_name object.

This macro casts pobject to a pointer to a class_name object. pobject must either be NULL, or point to an object of a class which is derived directly, or indirectly, from class_name. In builds of your application with the _DEBUG preprocessor symbol defined, the macro will ASSERT if pobject is not NULL and if it points to an object that is not a "kind of" the class_name (see definition for the CObject::IsKindOf). In non- _DEBUG builds, the macro performs the cast without any type checking.

class_name must be derived from CObject and use the DECLARE_DYNAMIC and IMPLEMENT_DYNAMIC, the DECLARE_DYNCREATE and IMPLEMENT_DYNCREATE, or the DECLARE_SERIAL and IMPLEMENT_SERIAL macros explained in the Help topic "CObject Class: Deriving a Class from Cobject."

For example, you might cast a pointer to CYourDocument, called pYourDoc, to a pointer to CDocument using the following expression:

      CDocument* pDoc = STATIC_DOWNCAST(CDocument, pYourDoc);

If pYourDoc does not point to an object derived directly or indirectly from CDocument, the macro will ASSERT.

Similarly, the documentation on DYNAMIC_DOWNCAST should be supplemented by saying that class_name must be derived from CObject and use the DECLARE_DYNAMIC and IMPLEMENT_DYNAMIC, the DECLARE_DYNCREATE and IMPLEMENT_DYNCREATE, or the DECLARE_SERIAL and IMPLEMENT_SERIAL macros.

If you leave out the pair of DECLARE_... and IMPLEMENT_... macros from your code, trying to cast the class pointer using STATIC_DOWNCAST or DYNAMIC_DOWNCAST gives compiler errors like the following:

error C2039: 'classCMyClass' : is not a member of 'CMyClass'

error C2065: 'classCMyClass' : undeclared identifier

MORE INFORMATION

The documentation on CObject::IsKindOf should also include:

This function works only for classes declared with the
DECLARE_DYNAMIC, DECLARE_DYNCREATE, or DECLARE_SERIAL macros.

The documentation on DECLARE_DYNCREATE should also include:

The DECLARE_DYNCREATE macro includes all the functionality of
DECLARE_DYNAMIC.

You may also want to see the topics on the static_cast and dynamic_cast operators in the Visual C++ documentation. Please note that the STATIC_DOWNCAST and DYNAMIC_DOWNCAST macros suggest an action that is contrary to the common usage of the term "downcast". The term "downcast" is commonly used to signify the movement of an object down a class hierarchy, from a given class to a class derived from it. This may be confusing when you consider that the STATIC_DOWNCAST and DYNAMIC_DOWNCAST macros perform the action of "upcasting". The term "upcast" is commonly used to signify the movement of an object up the class hierarchy, from a derived class to a class it is derived from.

REFERENCES

FIX: STATIC_DOWNCAST and DYNAMIC_DOWNCAST

SYMPTOMS

STATUS

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