Ruby Language Features

Iteration

Two Ruby features that are a bit unlike what you may have seen before, and which take some getting used to, are “blocks” and iterators. Instead of looping over an index (like with C, C++, or pre-1.5 Java), or looping over a list (like Perl’s for (@a) {...}, or Python’s for i in aList: ...), with
Ruby you’ll very often instead see

some_list.each do this_item
  # We're inside the block.
  # deal with this_item.
end

For more info on each (and it’s friends collect, find, inject, sort, etc.), see ri Enumerable (and then ri Enumerable#func_name).

Everything has a value

There’s no difference between an expression and a statement. Everything has a value, even if that value is nil. This is possible:

x = 10
y = 11
z = if x < y
  true
else
  false
end
z # => true

Symbols are not lightweight Strings

Many Ruby newbies struggle with understanding what Symbols are, and what they can be used for. Symbols can best be described as identities. A symbol is all about who it is, not what it is. Fire up
irb and see the difference:

irb(main):001:0> :george.object_id == :george.object_id
=> true
irb(main):002:0> "george".object_id == "george".object_id
=> false
irb(main):003:0>

The object_id methods returns the identity of an Object. If two objects have the same object_id, they are the same (point to thesame Object in memory).

As you can see, once you have used a Symbol once, any Symbol with the same characters references the same Object in memory. For any given two Symbols that represent the same characters, the object_ids match.

Now take a look at the String (“george”). The object_ids don’t match. That means they’re referencing two different objects in memory. Whenever you use a new String, Ruby allocates memory for it.

If you’re in doubt whether to use a Symbol or a String, consider what’s more important: the identity of an object (i.e. a Hash key), or the contents (in the example above, “george”).

Everything is an Object

“Everything is an object” isn’t just hyperbole. Even classes and integers are objects, and you can do the same things with them as with any other object:

# This is the same as
# class MyClass
#   attr_accessor :instance_var
# end
MyClass = Class.new do
  attr_accessor :instance_var
end

Variable Constants

Constants are not really constant. If you modify an already initialized constant, it will trigger a warning, but not halt your program. That isn’t to say you should redefine constants, though.

Naming conventions

Ruby enforces some naming conventions. If an identifier starts with a capital letter, it is a constant. If it starts with a dollar sign ($), it is a global variable. If it starts with @, it is an instance variable. If it
starts with @@, it is a class variable.

Method names, however, are allowed to start with capital letters. This can
lead to confusion, as the example below shows:

Constant = 10
def Constant
  11
end

Now Constant is 10, but Constant() is 11.

Fake keyword parameters

Ruby doesn’t have keyword parameters, like Python has. However, it can be
faked by using symbols and hashes. Ruby on Rails, among others, uses this
heavily. Example:

def some_keyword_params( params )
  params
end
some_keyword_params( :param_one => 10, :param_two => 42 )
# => {:param_one=>10, :param_two=>42}

The universal truth

In Ruby, everything except nil and false is
considered true. In C, Python and many other languages, 0 and possibly other
values, such as empty lists, are consided false. Take a look at the following
Python code (the example applies to other languages, too):

# in Python
if 0:
  print "0 is true"
else:
  print "0 is false"

This will print “0 is false”. The equivalent Ruby:

# in Ruby
if 0
  puts "0 is true"
else
  puts "0 is false"
end

Prints “0 is true”.

Access modifiers apply until the end of scope

In the following Ruby code,

class MyClass
  private
  def a_method; true; end
  def another_method; false; end
end

You might expect another_method to be public. Not so. The
‘private’ access modifier continues until the end of the scope, or until another
access modifier pops up, whichever comes first. By default, methods are
public:

class MyClass
  # Now a_method is public
  def a_method; true; end

  private

  # another_method is private
  def another_method; false; end
end

public,
private and protected are really methods, so they can
take parameters. If you pass a Symbol to one of them, that method’s visibility
is altered.

Method access

In Java, public means a method is accessible by anyone.
protected means the class’s instances and their ancestors can
access it, but not anyone else, and private means nobody besides
the classes instances can access the method.

Ruby differs slightly. public is, naturally, public.
private means the method(s) are accessible only when they can be
called without an explicit receiver. Only self is allowed to be
the receiver of a private method call.

protected is the one to
be on the lookout for. A protected method can be called from a class or its
ancestors’ instances, but also with another instance as its receiver. Example,
adapted from the Ruby
FAQ :

$ irb
irb(main):001:0> class Test
irb(main):002:1>   def func
irb(main):003:2>     99
irb(main):004:2>   end
irb(main):005:1>
irb(main):006:1*   def ==(other)
irb(main):007:2>     func == other.func
irb(main):008:2>   end
irb(main):009:1> end
=> nil
irb(main):010:0>
irb(main):011:0* t1 = Test.new
=> #<Test:0xb7db7bbc>
irb(main):012:0> t2 = Test.new
=> #<Test:0xb7daf688>
irb(main):013:0> t1 == t2
=> true
irb(main):014:0> # Now make 'func' private
irb(main):015:0* class Test
irb(main):016:1>   private :func
irb(main):017:1> end
=> Test
irb(main):018:0>
irb(main):019:0* t1 == t2
NoMethodError: private method 'func' called for
#<Test:0xb7db7bbc>
        from (irb):7:in '=='
        from (irb):19
        from :0
irb(main):020:0>

Classes are open

Ruby classes are open. You can open them up, add to
them, and change them at any time. Even core classes, like Fixnum
or even Object, the parent of all objects. Ruby on Rails defines a
bunch of methods for dealing with time on Fixnum. Watch:

class Fixnum
  def hours
    self * 3600 # number of seconds in an hour
  end
  alias hour hours
end

# 14 hours from 00:00 January 1st
# (aka when you finally wake up ;)
Time.mktime(2006, 01, 01) + 14.hours # => Sun Jan 01 14:00:00

Funny method names

In Ruby, methods are allowed to end with question
marks or exlamation marks. By convention, methods that answer questions (i.e.
Array#empty? returns true if the receiver is empty)
end in question marks. Potentially “dangerous” methods (ie methods that modify
self or the arguments, exit! etc.) by convention
end with exclamation marks.

All methods that change their arguments don’t end with exclamation marks,
though. Array#replace replaces the contents of an array with the
contents of another array. It doesn’t make much sense to have a method like that
that doesn’t modify self.

Singleton methods

Singleton methods are per-object methods. They are only available on the
Object you defined it on.

class Car
  def inspect
    "Cheap car"
  end
end

porsche = Car.new
porsche.inspect # => Cheap car
def porsche.inspect
  "Expensive car"
end

porsche.inspect # => Expensive car

# Other objects are not affected
other_car = Car.new
other_car.inspect # => Cheap car

Missing methods

Ruby doesn’t give up if it can’t find a method that responds to a particular
message. It calls the method_missing method with the name of the
method it couldn’t find and the arguments. By default, method_missing raises a
NameError exception, but you can redefine it to better fit your application, and
many libraries do. Here is an example:

# id is the name of the method called, the * syntax collects
# all the arguments in an array named 'arguments'
def method_missing( id, *arguments )
  puts "Method #{id} was called, but not found. It has " +
       "these arguments: #{arguments.join(", ")}"
end

__ :a, :b, 10
# => Method __ was called, but not found. It has these
# arguments: a, b, 10

The code above just prints the details of the call, but you are free to
handle the message in any way that is appropriate.

Message passing, not function calls

A method call is really a message to another object:

# This
1 + 2
# Is the same as this ...
1.+(2)
# Which is the same as this:
1.send "+", 2

Blocks are Objects, they just don’t know it yet

Blocks (closures, really) are heavily used by the standard library. To call a
block, you can either use yield, or make it a Proc by
appending a special argument to the argument list, like so:

def block( &the_block )
  # Inside here, the_block is the block passed to the method
  the_block # return the block
end
adder = block { a, b a + b }
# adder is now a Proc object
adder.class # => Proc

You can create blocks outside of method calls, too, by calling Proc.new with
a block or calling the lambda method.

Similarly, methods are also Objects in the making:

method(:puts).call "puts is an object!"
# => puts is an object!

Operators are syntactic sugar

Most operators in Ruby are just syntactic sugar (with some precedence rules)
for method calls. You can, for example, override Fixnums + method:

class Fixnum
  # You can, but please don't do this
  def +( other )
    self - other
  end
end

You don’t need C++’s operator+, etc.

You can even have
array-style access if you define the [] and []=
methods. To define the unary + and – (think +1 and -2), you must define the
+@ and -@ methods, respectively.

The operators below are not syntactic sugar, though. They
are not methods, and cannot be redefined:

=, .., ..., !, not, &&, and, , or, !=, !~, ::

In
addition, +=, *= etc. are just abbrevations for var = var +
other_var, var = var * other_var, etc. and therefore cannot
be redefined.