When to use Struct instead of Hash in Ruby?

TK. picture TK. · Jul 18, 2010 · Viewed 18.3k times · Source

I don't have much programming experience. But, to me, Struct seems somewhat similar to Hash.

  • What can Struct do well?
  • Is there anything Struct can do, that Hash cannot do?

After googling, the concept of Struct is important in C, but I don't know much about C.

Answer

sepp2k picture sepp2k · Jul 18, 2010

Structs differ from using hashmaps in the following ways (in addition to how the code looks):

  • A struct has a fixed set of attributes, while you add new keys to a hash.
  • Calling an attribute that does not exist on an instance of a struct will cause a NoMethodError, while getting the value for a non-existing key from a hash will just return nil.
  • Two instances of different structs will never be equal even if the structs have the same attributes and the instances have the same values (i.e. Struct.new(:x).new(42) == Struct.new(:x).new(42) is false, whereas Foo = Struct.new(:x); Foo.new(42)==Foo.new(42) is true).
  • The to_a method for structs returns an array of values, while to_a on a hash gets you an array of key-value-pairs (where "pair" means "two-element array")
  • If Foo = Struct.new(:x, :y, :z) you can do Foo.new(1,2,3) to create an instance of Foo without having to spell out the attribute names.

So to answer the question: When you want to model objects with a known set of attributes, use structs. When you want to model arbitrary use hashmaps (e.g. counting how often each word occurs in a string or mapping nicknames to full names etc. are definitely not jobs for a struct, while modeling a person with a name, an age and an address would be a perfect fit for Person = Struct.new(name, age, address)).

As a sidenote: C structs have little to nothing to do with ruby structs, so don't let yourself get confused by that.