I first read of this idiom in JimCoplien'''''''s now famous work AdvancedCeePlusPlusProgrammingStylesAndIdioms. Even before then, it was ubiquitous, though not well documented, in various reference counted classes such as C++ string classes. It is now the basic idiom used by almost all smart pointer classes with ExclusiveOwner or SharedOwner semantics. Later, this idiom was specialized by the GangOfFour into two StructuralPatterns, the AdapterPattern and the BridgePattern.

Envelope/Letter is an extension of the HandleBodyPattern, with multiple implementation (body/letter) classes derived from the interface (handle/envelope) class. 

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CategoryPattern