What are the various meanings of the Ruby sharp/number sign/pound/hash(#) symbol
How many contexts does the symbol # in Ruby have ?
I know that #` represents comment
# a comment
or 'convert to the value':
i = 1
print "#{i}" # simple example
However I also see some Ruby docs describe built-in methods like these:
Array#fill
File::file?
Why do they describe the same thing using 2 different symbols ? I am new in Ruby. Thanks
This is how instance method described:
Array#fill
So you can:
a = Array.new(2)
=> [nil, nil]
a.fill(42)
=> [42, 42]
This is how class method described:
String::new
s = String.new('abc')
=> "abc"
In Perl, # is used for commenting, and since Perl is an 'ancestor' of Ruby, the role was carried over.
The "#{}" syntax is called 'interpolation' and the pound was picked most likely because interpolation is similar in a sense to commenting, because you are changing the context of your code (in this case to another context for execution)
The # following a Class name is just meant to indicate the following identifier is a method of that Class, and is just a convention. Read more about it here: Why are methods in Ruby documentation preceded by a hash sign?
The :: is interesting, it acts similarly to the . in that you can call methods via both
Car::is_hybrid?
and
car.is_hybrid?
As you will see in most code though . is preferred for methods.
One case where :: is often preferred is where you have constant in the class and you will see this in system calls such as Math::PI or ones you create, e.g. ThePentagon::NUMBER_OF_BUILDING_SIDES
Just to show you as an example,that Ruby shows instance method preceded with the symbol # and class methods preceded with the symbol ..
class Foo
def self.foo;end
def bar;end
end
p Foo.method(:foo) # => #<Method: Foo.foo>
p Foo.new.method(:bar) # => #<Method: Foo#bar>
