Question

I want to set an object to ‘nil’ as I enumerate through an array, as follows:

for(Object* object in array){
    object = nil;
}

Xcode then tells me ‘Fast enumeration variables can’t be modified in ARC by default; declare the variable __strong to allow this.’

Which means doing this:

for(Object __strong* object in array){
    object = nil;
}

This seems to be redundant. As far as I understand, declaring a strong reference to an object increases its retain count by one, and nil-ing it decreases the retain count by one. So how, then, do I set an object to nil while enumerating through an array?

I am using ARC.

Answer 1

See Fast enumeration iteration variables in the Clang “Objective-C Automatic Reference Counting” documentation:

If a variable is declared in the condition of an Objective-C fast
enumeration loop, and the variable has no explicit ownership
qualifier, then it is qualified with const __strong and objects
encountered during the enumeration are not actually retained.

Rationale
This is an optimization made possible because fast
enumeration loops promise to keep the objects retained during
enumeration, and the collection itself cannot be synchronously
modified. It can be overridden by explicitly qualifying the variable
with __strong, which will make the variable mutable again and cause
the loop to retain the objects it encounters.

So by default, the loop variable is immutable, and the retain count of the current object is not increased for performance reasons.

If you explicitly declare the loop variable as __strong, it is a mutable strong reference, and the retain count of the current object is increased, and setting the loop variable to nil decreases the retain count again. But doing so does not deallocate the object or remove it from the array, because the array holds another strong reference to the object.