Where to use atomic in Objective C ?

Atomic is a property in objective C. It Is a default behaviour. It will ensure the present process is completed by the cpu, before another process access the variable. It is not fast, as it ensures the process is completed entirely.

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atomic's usefulness is largely limited only to situations where you need to access or set a particular value from multiple threads where that value is also integral

Beyond a single value, atomic cannot be used for thread safety purposes.
if the instance variable is gonna be accessed on multi threading environment.