CODEWITHSUNDEEP
rubystruct
Leo Le
Leo Le

Reputation: 91

Struct with and without member

I'm a new Rubyist, currently I'm going to use the Struct in class in order to create a temporary object. However, I encounter a question when I use a member in Struct like this:

class A
  attr_accessor :b
  B = Struct.new(:name, :print_name) do
    def print_name
      p "Hello #{name}"
    end
  end

  def initialize(input_name)
    @b = B.new(input_name)
  end
end

a = A.new('Leo')
a.b.print_name # Hello Leo

But I also receive the same result when my Struct B params don't include :print_name.

B = Struct.new(:name) do
...
end

So what is the different ? And when should I use the member param and when I'm not ?

Thanks

Upvotes: 4

Views: 964

Answers (1)

Andrey Deineko
Andrey Deineko

Reputation: 52357

In first case you define a class which's initializer takes two arguments - name and print_name.

In second case you define a class which's initializer takes single argument - name.

These has nothing to do with the fact, that you are defining an instance method called print_name.

So instances of both classes (with and without print_name argument) have print_name method defined, thus both examples work identically.

The difference will be visible when you inspect created objects:

# first case with two arguments
foo = B.new(:a, :b)
foo.inspect
=> "#<struct B name=:a, print_name=:b>"

# second case with single argument
foo = B.new(:a)
foo.inspect
=> "#<struct B name=:a>"

Also, when you'll check instance methods of B class for both cases, you can see the difference:

# first case with two arguments
B.instance_methods false
#=> [:name, :name=, :print_name, :print_name=]

# second case with single argument
B.instance_methods false
#=> [:name, :name=, :print_name]

But I also receive the same result when my Struct B params don't include :print_name

The difference, is that in first case you are able to do the following:

a.b.print_name = 'new print name'
a.b.inspect
#=> "#<struct B name='Leo', print_name='new print name'>"

Whereas in second case it will fail:

a.b.print_name = 'new print name'
#=> NoMethodError: undefined method 'print_name=' for #<struct B name='Leo'>

Upvotes: 3

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