What do non-ARC Objective-C property accessors look like?

Question

I want to know how the getter and setter for an Objective-C property are implemented as part of learning the concept of memory management.

I have not been able to find an actual representation other than "nonatomic" and "atomic".

What does the actual code look like for getters and setters with the different property attributes, such as strong/weak, copy/assign, and __unsafe_unretained?

Answer 1

You can check the source code at the objc4 github repo

getter: https://github.com/opensource-apple/objc4/blob/cd5e62a5597ea7a31dccef089317abb3a661c154/runtime/objc-accessors.mm#L48

id objc_getProperty_non_gc(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic) {
    if (offset == 0) {
        return object_getClass(self);
    }

    // Retain release world
    id *slot = (id*) ((char*)self + offset);
    if (!atomic) return *slot;

    // Atomic retain release world
    spinlock_t& slotlock = PropertyLocks[slot];
    slotlock.lock();
    id value = objc_retain(*slot);
    slotlock.unlock();

    // for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
    return objc_autoreleaseReturnValue(value);
}

setter: https://github.com/opensource-apple/objc4/blob/cd5e62a5597ea7a31dccef089317abb3a661c154/runtime/objc-accessors.mm#L70

static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
    if (offset == 0) {
        object_setClass(self, newValue);
        return;
    }

    id oldValue;
    id *slot = (id*) ((char*)self + offset);

    if (copy) {
        newValue = [newValue copyWithZone:nil];
    } else if (mutableCopy) {
        newValue = [newValue mutableCopyWithZone:nil];
    } else {
        if (*slot == newValue) return;
        newValue = objc_retain(newValue);
    }

    if (!atomic) {
        oldValue = *slot;
        *slot = newValue;
    } else {
        spinlock_t& slotlock = PropertyLocks[slot];
        slotlock.lock();
        oldValue = *slot;
        *slot = newValue;        
        slotlock.unlock();
    }

    objc_release(oldValue);
}

load weak variable: https://github.com/opensource-apple/objc4/blob/cd5e62a5597ea7a31dccef089317abb3a661c154/runtime/NSObject.mm#L444

id
objc_loadWeakRetained(id *location)
{
    id result;

    SideTable *table;

 retry:
    result = *location;
    if (!result) return nil;

    table = &SideTables()[result];

    table->lock();
    if (*location != result) {
        table->unlock();
        goto retry;
    }

    result = weak_read_no_lock(&table->weak_table, location);

    table->unlock();
    return result;
}

I don't think there is any code for unsafe_retained. Compiler can just simply assign the pointer without anything else.