Are strings mutable in Ruby?

Just to complement, one implication of this mutability is seem below:

ruby-1.9.2-p0 :001 > str = "foo"
 => "foo" 
ruby-1.9.2-p0 :002 > ref = str
 => "foo" 
ruby-1.9.2-p0 :003 > str = str + "bar"
 => "foobar" 
ruby-1.9.2-p0 :004 > str
 => "foobar" 
ruby-1.9.2-p0 :005 > ref
 => "foo" 

and

ruby-1.9.2-p0 :001 > str = "foo"
 => "foo" 
ruby-1.9.2-p0 :002 > ref = str
 => "foo" 
ruby-1.9.2-p0 :003 > str << "bar"
 => "foobar" 
ruby-1.9.2-p0 :004 > str
 => "foobar" 
ruby-1.9.2-p0 :005 > ref
 => "foobar" 

So, you should choose wisely the methods you use with strings in order to avoid unexpected behavior.

Also, if you want something immutable and unique throughout your application you should go with symbols:

ruby-1.9.2-p0 :001 > "foo" == "foo"
 => true 
ruby-1.9.2-p0 :002 > "foo".object_id == "foo".object_id
 => false 
ruby-1.9.2-p0 :003 > :foo == :foo
 => true 
ruby-1.9.2-p0 :004 > :foo.object_id == :foo.object_id
 => true 

Yes, << mutates the same object, and + creates a new one. Demonstration:

irb(main):011:0> str = "hello"
=> "hello"
irb(main):012:0> str.object_id
=> 22269036
irb(main):013:0> str << " world"
=> "hello world"
irb(main):014:0> str.object_id
=> 22269036
irb(main):015:0> str = str + " world"
=> "hello world world"
irb(main):016:0> str.object_id
=> 21462360
irb(main):017:0>