What does public, private & protected mean in Ruby?
These 3 methods control the public interface of your class.
They control WHO can call these methods.
By default all your methods are public, anyone can use them.
But you can restrict them to internal use only.
Why would you want to do that?
To make these internal methods easier to change later.
Imagine you’re writing a code library that is going to be used in a few different projects at work.
When these projects implement your library they’re going to be calling methods on the classes that you’ve created.
You release a new version…
But you decided to change the name on a public method.
That’s going to produce errors on every project that is using this method!
By having LESS public methods you INCREASE the freedom of change inside your class.
Let’s see some code examples.
Public is the default method visibility in Ruby.
Here’s an example:
def orange "Vitamin C" end
If you have an object
food that defines
orange, you can call it like this:
If a method has been made private or protected, you can make it public again.
If you’ve ever seen the “private method called” error message then you’ve tried to use a private method.
You can only use a private method directly.
Without an object, like this:
self.puts 123 # NoMethodError: private method `puts' called
It’s the same method, but because it’s declared as private you can only call it directly.
You can only use private methods:
In other words, you can’t call private methods from outside the class that defines them.
How do you define a private method?
def bacon "private bacon" end private :bacon
If you want to declare multiple methods as private…
class Food def public_method end private def bacon end def orange end def coconut end end
Every method after
private becomes a private method.
It’s a common pattern to define all your public methods first, then define your
private methods together at the end of the class.
Protected methods are way more rare…
They are like
private methods, but you can call them on an object & not just directly.
If you try this example with private you’ll get an error:
class Food def initialize(name) @name = name end def ==(other) name == other.name end protected attr_reader :name end food = Food.new("chocolate") puts food == food
You get the error because
name would be private so you can’t do
protected this code works!
That’s the difference, the fact that
protected keeps the method
private, but it also allows you to call that method on an object.
With private you can only do
name, with protected you can do
When should you use protected?
Only if you have a very specific case, like the equals (
The Ruby documentation recommends using
private instead of
protected whenever possible.
And there’s this note:
“Note that a protected method is slow because it can’t use inline cache.”
I was curious about this so I ran some benchmarks:
public: 2813891.9 i/s private: 2699273.8 i/s protected: 2572122.0 i/s
That’s a difference of 8.5% in performance.
You’ve learned about Ruby method visibility, public, private & protected methods. These aren’t Ruby keywords, they are methods themselves defined on the
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