Question

I am listening/watching the tutorial on Paul Hegarty from the App Store. In his lesson he states that you should ALWAYS synthesize your properties on the implementation file like so:

@sysnthesize example = _example;

I am also doing a apple documentation tutorial that does not synthesize properties. It also has init methods like so:

- (id)initWithName:(NSString *)name location:(NSString *)location date:(NSDate *)date
{
    self = [super init];

    if (self)
    {
        _name = name;

        _location = location;

        _date = date;

        return self;
    }

    return nil;
}

@end

Will these to interact, cancel or otherwise mess with each other if add them together like so:

@implementation BirdSighting

@synthesize name = _name;

@synthesize location = _location;

@synthesize date = _date;

- (id)initWithName:(NSString *)name location:(NSString *)location date:(NSDate *)date
{
    self = [super init];

    if (self)
    {
        _name = name;

        _location = location;

        _date = date;

        return self;
    }

    return nil;
}

@end

Thanks for the help.

Answer 1

Thinking of it in terms of “will these interact” is perhaps not the right mental model. The place to start is understanding what each of those lines does. So let’s consider one from each example:

@synthesize name = _name;

If a property (@property, usually in the .h) called name exists, then this does a few things:

1. It creates two methods, called -setName: and -name, which set and get the instance variable _name
2. If an instance variable called _name doesn’t exist, it creates one

Now the other example:

_name = name;

This does one thing; it assigns the address of ‘name’ to the instance variable _name.

So now that we understand what those do, let’s consider how they interact:

1. If there’s no explicit instance variable called _name to assign to, then the second line will not compile. The first line is one way to make sure there is an instance variable of that name
2. If you set the name property (either via dot notation or the -setName methods, it makes no difference), then that will modify the same _name instance variable, by means of the methods created by the @synthesize statement
3. If you use _name in your code, or if you use dot notation or the -name method, it will have the value most recently assigned to it by either of these means.
4. Doing _name = name rather than self.name = name; will NOT call the -setName: method, which means anything it does besides set (for example: retaining the argument) will not happen.

One additional consideration:
In very recent versions of Xcode, the compiler will automatically put the @synthesize statement in for you if you leave it out. This is a good thing (less typing, less redundancy, less things to get wrong), but it could be confusing if you’re not expecting it. For example, my point 1 above will not apply with new versions.