CODEWITHSUNDEEP
ruby
BigD
BigD

Reputation: 75

How does this object construction with struct work internally?

Struggling to understand how an object can be created this way. What is happening behind the scenes here when constructing this object ?

class UserPreview < Struct.new(:gid, :email, :name)
end

u = UserPreview.new(1, '[email protected]', 'Hans Peter')
=> #<struct UserPreview gid=1, email="[email protected]", name="Hans Peter">

Upvotes: 3

Views: 110

Answers (1)

Christopher Oezbek
Christopher Oezbek

Reputation: 26473

I think there are three things to discuss in your code:

# 1
Struct.new(:gid, :email, :name)

# 2
class UserPreview < OtherClass
end

# 3
u = UserPreview.new(1, '[email protected]', 'Hans Peter')

Starting with # 2:

UserPreview is created as a standard sub-class from OtherClass (in this case the result of the Struct.new call). We see the sub-class is not adding any new methods or attributes so it is essentially the same class as OtherClass with a different name. In Ruby you could simplify this as:

UserPreview = OtherClass

Because classes are just objects (of class Class).

Let's look at # 1:

What happens here is that we construct a new Struct instance with the given parameters. Under hood Struct is manually implemented in Ruby using C. You can look at the code on Github. I think nothing particular surprising happens. The given symbols are used to create a new class (either anonymously or named) and set the members provided (see here)

Last let's look at # 3:

# 3
u = UserPreview.new(1, '[email protected]', 'Hans Peter')

In this case the C function rb_struct_initialize_m is called. Since you provided just a list of arguments the code just walks the member fields and set each one in turn:

for (long i=0; i<argc; i++) {
  RSTRUCT_SET(self, i, argv[i]);

Upvotes: 1

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