What do we have:

A singleton class where we can initialise a string pattern. We then call a singleton function that doubles the pattern before printing it on console.

The Problem:

Instead of a string pattern, the console prints (null).

Concepts:

Who hasn’t faced a question about design patterns in an interview? This problem can be followed after a general enquiry about the common design patterns. As most people have the singleton code memorised, we can ask the candidate to write the alternate code snippet for dispatch_once and then strike up a discussion about GCD functions.

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