FAQ 34.08 Why does COM need GUIDs (and CLSIDs and IIDs)?
In C++, classes have names and the name of a class must be unique in the context of a linked program (otherwise the compiler or the linker will detect the duplicate names and report an error). Namespaces have been added to C++ to reduce the chances of name conflicts, but they don't eliminate the possibility of name conflicts. If there are name conflicts, you have to come up with some way to make all the class names unique before you can get your program to compile and link. The situation is slightly more complex with COM. With COM, you have thousands of developers working around the world and defining thousands of components that are delivered to users as binary components. Imagine if COM used nice short text names (like C++ does) for class names and interface names. How many COM classes and COM interfaces, worldwide, do you think would be called String? List? Date? So, the chances of name clashes are high. Furthermore, COM classes are delivered to you as binary components, so you can't modify them to resolve name conflicts. After all, what are you going to do? Edit the binary? That seems a little extreme. Or maybe you could call the vendor and have them change the name of the class and ship you a new version? That's impractical. So the solution is to have names that are 128 bits long (giving a namespace of 2128 unique names) and to employ a method that makes sure that everyone generates different names. One of these methods is to have programmers use the utility program guidgen.exe to generate GUIDs. This program generates GUIDs based on unique information built into the network card of the computer. Another method is to call the COM routine CoCreateGuid to generate new GUIDs (there are some other methods we won't discuss here, but you get the general idea). Of course these methods can be compromised, but in practice, the likelihood of two developers creating duplicate GUIDs is so small that no one worries about it. |
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