Reads and returns a line from the stream;
returns +nil+ if at end-of-stream.

Side effects:

- Increments stream position by the number of bytes read.
- Assigns the return value to global variable <tt>$_</tt>.

With no arguments given, reads a line using the default record separator
(global variable <tt>$/</tt>,* whose initial value is <tt>"\n"</tt>):

  strio = StringIO.new(TEXT)
  strio.pos  # => 0
  strio.gets # => "First line\n"
  strio.pos  # => 11
  $_         # => "First line\n"
  strio.gets # => "Second line\n"
  strio.read # => "\nFourth line\nFifth line\n"
  strio.eof? # => true
  strio.gets # => nil

  strio = StringIO.new('こんにちは')  # Five 3-byte characters.
  strio.pos  # => 0
  strio.gets # => "こんにちは"
  strio.pos  # => 15

<b>Argument +sep+</b>

With only string argument +sep+ given, reads a line using that string as the record separator:

  strio = StringIO.new(TEXT)
  strio.gets(' ') # => "First "
  strio.gets(' ') # => "line\nSecond "
  strio.gets(' ') # => "line\n\nFourth "

<b>Argument +limit+</b>

With only integer argument +limit+ given,
reads a line using the default record separator;
limits the size (in characters) of each line to the given limit:

  strio = StringIO.new(TEXT)
  strio.gets(10) # => "First line"
  strio.gets(10) # => "\n"
  strio.gets(10) # => "Second lin"
  strio.gets(10) # => "e\n"

<b>Arguments +sep+ and +limit+</b>

With arguments +sep+ and +limit+ both given, honors both:

  strio = StringIO.new(TEXT)
  strio.gets(' ', 10) # => "First "
  strio.gets(' ', 10) # => "line\nSecon"
  strio.gets(' ', 10) # => "d "

<b>Position</b>

As stated above, method +gets+ reads and returns the next line in the stream.

In the examples above each +strio+ object starts with its position at beginning-of-stream;
but in other cases the position may be anywhere:

  strio = StringIO.new(TEXT)
  strio.pos = 12
  strio.gets # => "econd line\n"

The position need not be at a character boundary:

  strio = StringIO.new('こんにちは')  # Five 3-byte characters.
  strio.pos = 3                     # At beginning of second character.
  strio.gets  # => "んにちは"
  strio.pos = 4                     # Within second character.
  strio.gets  # => "\x82\x93にちは"

<b>Special Record Separators</b>

Like some methods in class IO, method +gets+ honors two special record separators;
see {Special Line Separators}[https://docs.ruby-lang.org/en/master/IO.html#class-IO-label-Special+Line+Separator+Values]:

  strio = StringIO.new(TEXT)
  strio.gets('')  # Read "paragraph" (up to empty line).
  # => "First line\nSecond line\n\n"

  strio = StringIO.new(TEXT)
  strio.gets(nil) # "Slurp": read all.
  # => "First line\nSecond line\n\nFourth line\nFifth line\n"

<b>Keyword Argument +chomp+</b>

With keyword argument +chomp+ given as +true+ (the default is +false+),
removes the trailing newline (if any) from the returned line:

  strio = StringIO.new(TEXT)
  strio.gets              # => "First line\n"
  strio.gets(chomp: true) # => "Second line"

Related: #each_line, #readlines,
{Kernel#puts}[rdoc-ref:Kernel#puts].
