Why is the shovel operator (<<) preferred over plus-equals (+=) when building a string in Ruby?

Question

I am working through Ruby Koans.

The test_the_shovel_operator_modifies_the_original_string Koan in about_strings.rb includes the follo

Answer 1

This is an old question, but I just ran across it and I'm not fully satisfied with the existing answers. There are lots of good points about the shovel << being faster than concatenation +=, but there is also a semantic consideration.

The accepted answer from @noodl shows that << modifies the existing object in place, whereas += creates a new object. So you need to consider if you want all references to the string to reflect the new value, or do you want to leave the existing references alone and create a new string value to use locally. If you need all references to reflect the updated value, then you need to use <<. If you want to leave other references alone, then you need to use +=.

A very common case is that there is only a single reference to the string. In this case, the semantic difference does not matter and it is natural to prefer << because of its speed.