How to Make Your Classes More Powerful by Implementing Equality
How do you compare two things in Ruby? Using == as you already know… but did you know that == is a method & not just syntax?
You can implement this method in your own classes to make them more powerful. And that’s what I want to talk about in Taxi Gatwick to London this post.
Equality Basics
As you know you can clinica kinetoterapie compare two strings like this:
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"foo" == "foo" |
And if the content is equal then this will evaluate to true. This works because the String class implements a == method that knows how to compare strings.
But what if String didn’t implement ==?
Then Ruby would use Object‘s implementation of ==, which defaults to testing for object identity, instead of object contents.
Example:
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Object.new == Object.new # false String.new == String.new # true |
Implementing Equality
Now let’s use what you just learned to make your own classes more powerful by being able to compare them.
Thanks to the == method you can define exactly what it means for two instances of your own class to be equal.
Example:
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class Product attr_reader :name, :price def initialize(name, price) @name, @price = name, price end def ==(other) self.name == other.name && self.price == other.price end end p1 = Product.new('book', 49) p2 = Product.new('book', 49) p1 == p2 # true |
The == method says that both the name and the price must be the same for two Product objects to be considered equal.
Remember:
If you don’t implement this method (or use the Comparable module, which I explain in my Ruby book) the two objects will be compared using their object id’s, instead of their values.
Also I should mention that if you use a Struct it already implements == for you.
What About Triple Equals?
You may be wondering if == is a method, is === also a method? And the answer is yes 🙂
So what’s the difference between the two?
In Javascript there is a clear difference, where == will try to convert the object types to be the same if they aren’t (1 vs '1'). And === is for ‘strict’ equality.
But in Ruby there is not such thing. What === means depends on the class implementing it.
In many cases it is just an alias for ==.
Like in String and Object.
Here’s a table of built-in classes which give === a special meaning:
| Class | Meaning |
|---|---|
| Range | Returns true if obj is an element of the range, false otherwise. |
| Regexp | Match regexp against a string. |
| Module | Returns true if obj is an instance of mod or and instance of one of mod’s descendants. |
| Proc | Invokes the block with obj as the proc’s parameter like Proc#call. It is to allow a proc object to be a target of a when clause in a case statement. |
Conclusion
In this post you learned how to make your classes more powerful by implementing the == method. You also learned the difference between == and ===.
Don’t forget to share this post so more people can see it 🙂