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	Chapters
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APPENDIX D. ENCAPSULATING STREAMS

All encapsulating streams must handle the :stream initarg, which is used to specify the stream
to be encapsulated.

=> standard-encapsulating-stream [Class]
This class provides a standard implementation of an encapsulating stream.

D.1.1 Encapsulating Stream Protocol

The standard-encapsulating-stream class must provide "trampoline" methods for all stream
protocol operations. These "trampolines" will simply call the same generic function on the
encapsulated stream. In particular, all of the generic functions in the following protocols must
have trampolines.

• The basic input and output stream protocols, as specified by the Gray stream proposal in
  Appendix C.
  
  
• The sheet protocols, as specified in Chapters 7 and 8.
  
  
• The medium protocol, as specified in Chapter 10.
  
  
• The text style binding forms, as specified in Chapter 11.
  
  
• The drawing functions, as specified in Chapter 12.
  
  
• The extended output stream protocol, as specified in Chapter 15.
  
  
• The output recording stream protocol, as specified in Chapter 16.
  
  
• The incremental redisplay stream protocol, as specified in Chapter 21.
  
  
• The extended input stream protocol, as specified in Chapter 22.
  
  
• The stream generics for presentation types, as specified in Chapter 23.
  

The following generic function must also be implemented for all encapsulating stream classes.
=> encapsulating-stream-stream encapsulating-stream [Generic Function]
Returns the stream encapsulated by the encapsulating stream encapsulating-stream.

D.1.2 The "Delegation Problem"

The suggested implementation of encapsulating streams has a potential problem that we label
the "delegation" or "multiple self" problem. Here is an example of the problem.

Suppose we implement accepting-values by using an encapsulating stream class called accepting-
values-stream that will be used to close over an ordinary extended input and output stream.

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Page 354	Table of Contents	Index	Page 356
	Chapters
	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 A, B, C, D, E