string-simple-stream

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Package: excl

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Allegro CL version 8.2
Unrevised from 8.1 to 8.2.
8.1 version

string-simple-stream

This kind of stream allows for the moving and possible translation of characters.

One subclass of string-simple-stream, composing-stream, allows composing of external-formats and ligatures using Java-style encapsulations.

Other subclasses of string-simple-stream have a single buffer which is in fact a string. It is available as an optimization to allow read and format at a high level to be used on strings. For resourcing, the string for the stream may be specified by a :string option to make-instance, which is then passed to device-open as an options flag, or the string for a string-output-simple-stream may be reused if it already exists in the stream.

A string stream that is not a composing-stream incorporates a major exception in the CL requirements for the streams system: there is no external connection; everything that lisp is reading from or writing to the stream is within the lisp heap, and thus there are no reconnect or shutdown problems with these streams; they continue to work across a dumped lisp image that has been restarted.

Specific exceptions to non-composing string-simple-streams operations:

The basic unit of transfer is the character, not the octet.
read-byte and write-byte will signal an error if given a string-simple-stream.
No external-format processing is performed on the characters read or written, since the characters do not become external, but remain within the lisp during the round trip through the stream.
Because CL specifies only input and output string streams (the classes string-input-simple-stream and string-output-simple-stream and perhaps further subclasses), but not bidirectional, it is not necessary to implement mode switching internally for the single string buffer. Implementations are free to define such a bidirectional string stream (or to emulate such a stream - see note just below).

Note: something similar to string streams could be implemented, with less efficient operation, by simulating an external-connection at the device level of either a single-channel or dual-channel stream. The "final destination" (or source) of the device would in that case be a string, and the extra octet buffering would require extra copying at the device level when transferring data out (or in). This technique would allow external-format processing on the strings either during input or output.