Question - What Is Dll Hell?
Answer -
1.Adding of a new virtual method to a class exported from a DLL can cause the following problems:
-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.
-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.
-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.