Answer / subhash - netedge computing

Overwriting a older version of dll by a new version of dll
is known as Dll Hell problem.

Answer / priya

1. Adding of a new virtual method to a class exported
from a DLL can cause the following problems:
o If class already has a virtual method B and we are
adding a new one named A before it, then we are changing
the table of virtual methods. Now the first virtual method
in the table will be A but not B and the client program
which calls B will fail without recompilation as the call
of B will cause the call of A and this is another method
which possibly has other parameters and return type.
o When a class doesn?t have virtual methods and none
of its base classes have, then adding of a new virtual
method to it or to its base class will cause adding of the
pointer for the table of virtual methods. This will cause
change in the class size. So the error will occur after a
client program will allocate memory for the class (an
amount of memory that was required before the class has
been changed) and will try to modify some of the class'
fields explicitly or implicitly. In this case, as the
pointer to the table of virtual method is added to the
beginning of the class, all addresses of the class fields
are shifted and thus we will get an abnormal client program
behavior or a runtime error.
o In case when a class has virtual methods or any of
its parent classes has, we can?t add virtual methods to
classes exported from the DLL if they participate in the
inheritance. We can?t add virtual methods not only to the
beginning of the class declaration but also to the end of
it. The problem is in shifting in the table of virtual
methods. Note that even if you add a new virtual method to
the end of the class declaration then the child?s virtual
methods will be shifted.
2. Adding of a new field (of any kind) to a class
declaration can cause following problems:
1. Adding of a new field to the end of a class causes
class size to change as in the case of adding of a virtual
method to a class that didn?t have any. The client program
will allocate less memory than it is required for a new
class and this will cause referencing to the memory out of
the class scope.
2. Adding of a new field between existing ones is even
worse. It will case all addresses of fields after the new
one to be shifted and a client application will work with
the incorrect addresses while working with the fields that
are situated after the new field. We also have the problem
as in the previous point here.

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