何时使用alloc init

Question

The following two examples each seem to work just fine. In example 1, widget is created before the while loop:

-(int)compareWidgets { // Example 1

    int count = 0;
    NSMutableArray *widgetsCopy = [self.widgets mutableCopy];

    Widgets *widget = [[Widgets alloc]init]; // HERE ? or...

    while (widgetsCopy.count) {
        widget = [widgetsCopy lastObject]; 
        [widgetsCopy removeLastObject];
        for (Widgets *compareWidget in widgetsCopy)
            if (compareWidget.value == widget.value)
                count += 1;
    }
    return count;

}

In example 2, widget is created inside the while loop...

-(int)compareWidgets { // Example 2

    int count = 0;
    NSMutableArray *widgetsCopy = [self.widgets mutableCopy];

    while (widgetsCopy.count) {
        Widgets *widget = [widgetsCopy lastObject]; // HERE ?
        [widgetsCopy removeLastObject];
        for (Widgets *compareWidget in widgetsCopy)
            if (compareWidget.value == widget.value)
                count += 1;
    }
    return count;

}

Q1: In the example 1, widget is allocated/initialized just once and then it seems to be reassigned for each iteration through the array. In example 2, alloc/init is never used but widget is, again, successfully assigned through each iteration. Why is this possible and which example is either correct or preferable?

Q2: Also, in either case, widget is assigned to an array object which is immediately removed from its array: [widgetsCopy removeLastObject] . Since the object widget points to has been removed from its array, why isn't widget nil at this point--why does it continue to retain the correct value?

Thanks.

Answer 1

The first code snippet creates an object with alloc / init , and then the body of the while immediately releases it.

There is no need to create an object and assign it to your variable in order to make that variable "valid": you can assign the variable as long as it is in scope at the time of the assignment.

Widget does not become nil right away, because under ARC variables are __strong by default, so an assignment to widget leads to the corresponding object being retained. If you change Widget *widget to __weak Widget *widget , you would see a nil there, but then there would be no point in declaring that variable in the first place.

Answer 2

Consider pointers as being an empty box. In the first example you put a new widget in that box. Then the very next thing you do is empty that box and put the lastObject in that box. The example breaks down a little since the widget is still in that array as well till you remove the last object. ARC lets you not have to worry as much about how many boxes it is in. Once it is taken out of the last box it will get released.