Upside-down inheritance is a technique for specifying the base class of base classes by using template arguments. MultipleInheritance is used heavily. Delegation is implemented via MixIn classes. http://www.apostate.com/programming/atlupsidedown.html (https://web.archive.org/web/20060315072824/http://www.apostate.com/programming/atlupsidedown.html)

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This mechanism is used to implement the ActiveTemplateLibrary and the WindowsTemplateLibrary on top of it.

The advantages over the MicrosoftFoundationClasses are:
* Small - only the code used in a final instantiation of your derived classes are compiled into your program, as opposed to the multi-megabyte MFC libraries and DLLs
* Fast - template instantiation rather than virtual methods are used to override behavior, allowing more aggressive compiler optimization
* Flexible - the existing class hierarchy can be redefined through the template arguments
** Complexity is the downside. The user must also be aware of dependencies between sibling classes.

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See also CuriouslyRecurringTemplate
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CategoryCpp CategoryCppTemplates