Question

i’m trying to understand this in objective-c :

in this example, indexPath is a pointer but we use it “as is” in the function : indexPath.section, instead of (for example) *indexPath.section(with a *) :

- (NSIndexPath *)tableView:(UITableView *)tableView willSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    return (indexPath.section == 0) ? nil : indexPath;
}

so, in objective-c, we don’t need to add a * to get the content of the variable where the pointer points to…?

but i found this function, where they use a * on the pointer inside the function (on reverse ) :

NSInteger lastNameFirstNameSort(id person1, id person2, void *reverse)
{
    NSString *name1 = [person1 valueForKey:LAST];

    //...

    if (*(BOOL *)reverse == YES) {
        return 0 - comparison;
    }

and for the id variables, they are using the variable name as is : for example here : person1

So, could someone explain me the differences between those 2 examples :
why on the first example, we don’t add a * on indexPath,
why we don’t add this * on the id variables, and we use it with *reverse in the second example?

Thanks

Answer 1

You are confusing dot-notation with reading a structure. This is not surprising, since Apple made them ambiguous.

indexPath.section does not mean “the section structure element in indexPath. It doesn’t even mean “the property section in indexPath.” It means [indexPath section]. It just calls the method section and returns the result.

Similarly, foo.bar = baz does not literally mean “set the property bar to baz.” It literally means [foo setBar:baz]. Whatever setBar: does will be done. In most cases, it sets an ivar.

Since indexPath is also technically a struct pointer, it is possible in some cases (but not always, and not often if you code correctly) to say indexPath->section. You should never do this. (There are some extremely rare exceptions, but you are unlikely to encounter them.)

The frustrating thing about all of this is that foo.bar might be a structure reference or it might be a method call. Without knowing what foo is, you can’t know. This is one of the problems with dot notation.

If you find it confusing, don’t use dot notation (it continues to be a controversial feature among experienced developers). It is never required. It’s just a shortcut for the more explicit [foo bar] and [foo setBar:baz].

*(BOOL *)result means “cast result from void* to BOOL * and then dereference it as a BOOL. It’s unrelated to dot notation.