@node Character and String Types
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@chapter Character and String Types

@menu
* Memory Layout::
* Reader and Printer::
@end menu

The @code{:SB-UNICODE} feature implies support for all 1114112 potential
characters in the character space defined by the Unicode consortium,
with the identity mapping between lisp @code{char-code} and Unicode code
point.  SBCL releases before version 0.8.17, and those without the
@code{:SB-UNICODE} feature, support only 256 characters, with the
identity mapping between @code{char-code} and Latin1 (or, equivalently,
the first 256 Unicode) code point.

In the absence of the @code{:SB-UNICODE} feature, the types
@code{base-char} and @code{character} are identical, and encompass the
set of all 256 characters supported by the implementation.  With the
@code{:SB-UNICODE} on @code{*features*} (the default), however,
@code{base-char} and @code{character} are distinct: @code{character}
encompasses the set of all 1114112 characters, while @code{base-char}
represents the set of the first 128 characters.

The effect of this on string types is that an SBCL configured with
@code{:SB-UNICODE} has three disjoint @code{string} types: @code{(vector
nil)}, @code{base-string} and @code{(vector character)}.  In a build
without @code{:SB-UNICODE}, there are two such disjoint types:
@code{(vector nil)} and @code{(vector character)}; @code{base-string} is
identically equal to @code{(vector character)}.

The @code{SB-KERNEL:CHARACTER-SET-TYPE} represents possibly
noncontiguous sets of characters as lists of range pairs: for example,
the type @code{standard-char} is represented as the type
@code{(sb-kernel:character-set '((10 . 10) (32 . 126)))}

@node Memory Layout
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@section Memory Layout

Characters are immediate objects (that is, they require no heap
allocation) in all permutations of build-time options.  Even on a 32-bit
platform with @code{:SB-UNICODE}, there are three bits to spare after
allocating 8 bits for the character widetag and 21 for the character
code.  There is only one such layout, and consequently only one widetag
is needed: the difference between @code{base-char} and @code{character}
is purely on the magnitude of the @code{char-code}.

Objects of type @code{(simple-array nil (*))} are represented in memory
as two words: the first is the object header, with the appropriate
widetag, and the second is the length field.  No memory is needed for
elements of these objects, as they can have none.
Such object are not strings though.

Objects of type @code{simple-base-string} have the header word
with widetag, then a word for the length, and after that a sequence of
8-bit @code{char-code} bytes.  The system arranges for there to be a
null byte after the sequence of lisp character codes.

Objects of type @code{(simple-array character (*))}, where this is a
distinct type from @code{simple-base-string}, have the header word with
widetag, length, and then a sequence of 32-bit @code{char-code} bytes.
Again, the system arranges for there to be a null word after the
sequence of character codes.

Non-simple character arrays, and simple character arrays of non-unit
dimensionality, have an array header with a reference to an underlying
data array of the appropriate form from the above representations.

@node Reader and Printer
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@section Reader and Printer

The @code{"} reader macro always constructs an object of type
@code{(simple-array character)}, even if all of the characters within
the quotation marks are of type @code{base-char}.  This implies that
only strings of type @code{(vector character)} will be able to be
printed when @code{*print-readably*} is true: attempting to print
strings of other types will cause an error of type
@code{print-not-readable}.

