[DOC] Main doc for encodings moved from encoding.c to doc/encodings.rdoc (#5748)

Main doc for encodings moved from encoding.c to doc/encodings.rdoc

doc/encodings.rdoc

@@ -1,4 +1,4 @@
-== \Encoding
+== Encodings
 
 === The Basics
 

doc/string/new.rdoc

@@ -12,7 +12,7 @@ with the same encoding as +string+:
   s.encoding # => #<Encoding:UTF-8>
 
 Literal strings like <tt>""</tt> or here-documents always use
-Encoding@Script+encoding, unlike String.new.
+{script encoding}[rdoc-ref:encodings.rdoc@Script+Encoding], unlike String.new.
 
 With keyword +encoding+, returns a copy of +str+
 with the specified encoding:

encoding.c

@@ -1936,203 +1936,33 @@ rb_enc_aliases(VALUE klass)
 }
 
 /*
- * An Encoding instance represents a character encoding usable in Ruby. It is
+ * An \Encoding instance represents a character encoding usable in Ruby.
- * defined as a constant under the Encoding namespace. It has a name and
+ * It is defined as a constant under the \Encoding namespace.
- * optionally, aliases:
+ * It has a name and, optionally, aliases:
  *
- *   Encoding::ISO_8859_1.name
+ *   Encoding::US_ASCII.name  # => "US-ASCII"
- *   #=> "ISO-8859-1"
+ *   Encoding::US_ASCII.names # => ["US-ASCII", "ASCII", "ANSI_X3.4-1968", "646"]
  *
- *   Encoding::ISO_8859_1.names
+ * A Ruby method that accepts an encoding as an argument will accept:
- *   #=> ["ISO-8859-1", "ISO8859-1"]
  *
- * Ruby methods dealing with encodings return or accept Encoding instances as
+ * - An \Encoding object.
- * arguments (when a method accepts an Encoding instance as an argument, it
+ * - The name of an encoding.
- * can be passed an Encoding name or alias instead).
+ * - An alias for an encoding name.
  *
- *   "some string".encoding
+ * These are equivalent:
- *   #=> #<Encoding:UTF-8>
  *
- *   string = "some string".encode(Encoding::ISO_8859_1)
+ *   'foo'.encode(Encoding::US_ASCII) # Encoding object.
- *   #=> "some string"
+ *   'foo'.encode('US-ASCII')         # Encoding name.
- *   string.encoding
+ *   'foo'.encode('ASCII')            # Encoding alias.
- *   #=> #<Encoding:ISO-8859-1>
  *
- *   "some string".encode "ISO-8859-1"
+ * For a full discussion of encodings and their uses,
- *   #=> "some string"
+ * see {the Encodings document}[rdoc-ref:encodings.rdoc].
  *
- * Encoding::ASCII_8BIT is a special encoding that is usually used for
+ * Encoding::ASCII_8BIT is a special-purpose encoding that is usually used for
- * a byte string, not a character string. But as the name insists, its
+ * a string of bytes, not a string of characters.
- * characters in the range of ASCII are considered as ASCII
+ * But as the name indicates, its characters in the ASCII range
- * characters.  This is useful when you use ASCII-8BIT characters with
+ * are considered as ASCII characters.
- * other ASCII compatible characters.
+ * This is useful when you use other ASCII-compatible encodings.
- *
- * == Changing an encoding
- *
- * The associated Encoding of a String can be changed in two different ways.
- *
- * First, it is possible to set the Encoding of a string to a new Encoding
- * without changing the internal byte representation of the string, with
- * String#force_encoding. This is how you can tell Ruby the correct encoding
- * of a string.
- *
- *   string
- *   #=> "R\xC3\xA9sum\xC3\xA9"
- *   string.encoding
- *   #=> #<Encoding:ISO-8859-1>
- *   string.force_encoding(Encoding::UTF_8)
- *   #=> "R\u00E9sum\u00E9"
- *
- * Second, it is possible to transcode a string, i.e. translate its internal
- * byte representation to another encoding. Its associated encoding is also
- * set to the other encoding. See String#encode for the various forms of
- * transcoding, and the Encoding::Converter class for additional control over
- * the transcoding process.
- *
- *   string
- *   #=> "R\u00E9sum\u00E9"
- *   string.encoding
- *   #=> #<Encoding:UTF-8>
- *   string = string.encode!(Encoding::ISO_8859_1)
- *   #=> "R\xE9sum\xE9"
- *   string.encoding
- *   #=> #<Encoding::ISO-8859-1>
- *
- * == Script encoding
- *
- * All Ruby script code has an associated Encoding which any String literal
- * created in the source code will be associated to.
- *
- * The default script encoding is Encoding::UTF_8 after v2.0, but it
- * can be changed by a magic comment on the first line of the source
- * code file (or second line, if there is a shebang line on the
- * first). The comment must contain the word <code>coding</code> or
- * <code>encoding</code>, followed by a colon, space and the Encoding
- * name or alias:
- *
- *   # encoding: UTF-8
- *
- *   "some string".encoding
- *   #=> #<Encoding:UTF-8>
- *
- * The <code>__ENCODING__</code> keyword returns the script encoding of the file
- * which the keyword is written:
- *
- *   # encoding: ISO-8859-1
- *
- *   __ENCODING__
- *   #=> #<Encoding:ISO-8859-1>
- *
- * <code>ruby -K</code> will change the default locale encoding, but this is
- * not recommended. Ruby source files should declare its script encoding by a
- * magic comment even when they only depend on US-ASCII strings or regular
- * expressions.
- *
- * == Locale encoding
- *
- * The default encoding of the environment. Usually derived from locale.
- *
- * see Encoding.locale_charmap, Encoding.find('locale')
- *
- * == Filesystem encoding
- *
- * The default encoding of strings from the filesystem of the environment.
- * This is used for strings of file names or paths.
- *
- * see Encoding.find('filesystem')
- *
- * == External encoding
- *
- * Each IO object has an external encoding which indicates the encoding that
- * Ruby will use to read its data. By default Ruby sets the external encoding
- * of an IO object to the default external encoding. The default external
- * encoding is set by locale encoding or the interpreter <code>-E</code> option.
- * Encoding.default_external returns the current value of the external
- * encoding.
- *
- *   ENV["LANG"]
- *   #=> "UTF-8"
- *   Encoding.default_external
- *   #=> #<Encoding:UTF-8>
- *
- *   $ ruby -E ISO-8859-1 -e "p Encoding.default_external"
- *   #<Encoding:ISO-8859-1>
- *
- *   $ LANG=C ruby -e 'p Encoding.default_external'
- *   #<Encoding:US-ASCII>
- *
- * The default external encoding may also be set through
- * Encoding.default_external=, but you should not do this as strings created
- * before and after the change will have inconsistent encodings.  Instead use
- * <code>ruby -E</code> to invoke ruby with the correct external encoding.
- *
- * When you know that the actual encoding of the data of an IO object is not
- * the default external encoding, you can reset its external encoding with
- * IO#set_encoding or set it at IO object creation (see IO.new options).
- *
- * == Internal encoding
- *
- * To process the data of an IO object which has an encoding different
- * from its external encoding, you can set its internal encoding. Ruby will use
- * this internal encoding to transcode the data when it is read from the IO
- * object.
- *
- * Conversely, when data is written to the IO object it is transcoded from the
- * internal encoding to the external encoding of the IO object.
- *
- * The internal encoding of an IO object can be set with
- * IO#set_encoding or at IO object creation (see IO.new options).
- *
- * The internal encoding is optional and when not set, the Ruby default
- * internal encoding is used. If not explicitly set this default internal
- * encoding is +nil+ meaning that by default, no transcoding occurs.
- *
- * The default internal encoding can be set with the interpreter option
- * <code>-E</code>. Encoding.default_internal returns the current internal
- * encoding.
- *
- *    $ ruby -e 'p Encoding.default_internal'
- *    nil
- *
- *    $ ruby -E ISO-8859-1:UTF-8 -e "p [Encoding.default_external, \
- *      Encoding.default_internal]"
- *    [#<Encoding:ISO-8859-1>, #<Encoding:UTF-8>]
- *
- * The default internal encoding may also be set through
- * Encoding.default_internal=, but you should not do this as strings created
- * before and after the change will have inconsistent encodings.  Instead use
- * <code>ruby -E</code> to invoke ruby with the correct internal encoding.
- *
- * == IO encoding example
- *
- * In the following example a UTF-8 encoded string "R\u00E9sum\u00E9" is transcoded for
- * output to ISO-8859-1 encoding, then read back in and transcoded to UTF-8:
- *
- *   string = "R\u00E9sum\u00E9"
- *
- *   open("transcoded.txt", "w:ISO-8859-1") do |io|
- *     io.write(string)
- *   end
- *
- *   puts "raw text:"
- *   p File.binread("transcoded.txt")
- *   puts
- *
- *   open("transcoded.txt", "r:ISO-8859-1:UTF-8") do |io|
- *     puts "transcoded text:"
- *     p io.read
- *   end
- *
- * While writing the file, the internal encoding is not specified as it is
- * only necessary for reading.  While reading the file both the internal and
- * external encoding must be specified to obtain the correct result.
- *
- *   $ ruby t.rb
- *   raw text:
- *   "R\xE9sum\xE9"
- *
- *   transcoded text:
- *   "R\u00E9sum\u00E9"
  *
  */