Editing DLL hell (section)

===Incompatible versions===
A particular version of a library can be compatible with some programs that use it and incompatible with others. Windows has been particularly vulnerable to this because of its emphasis on dynamic linking of C++ libraries and [[Object Linking and Embedding]] (OLE) objects. C++ classes export many methods, and a single change to the class, such as a new virtual method, can make it incompatible with programs that were built against an earlier version. Object Linking and Embedding has very strict rules to prevent this: interfaces are required to be stable, and memory managers are not shared. This is insufficient, however, because the semantics of a class can change. A bug fix for one application may result in the removal of a feature from another. Before [[Windows 2000]], Windows was vulnerable to this because the [[Component Object Model|COM]] class table was shared across all users and processes. Only one COM object in one DLL/EXE could be declared as having a specific global COM Class ID on a system. If any program needed to create an instance of that class, it got whatever was the current centrally registered implementation. As a result, an installation of a program that installed a new version of a common object might inadvertently break other programs that were previously installed.