Question

I am having a hard time on how to understand the actual use of setting a property to variables and objects. I tried to read many tutorials and apple documentation but still i am not able to understand on why we should use a property and why we need to synthesize the property? Considering we declare

@property(nonatomic,retain) NSString *str;

why do we need to declare a property. As far as i understand, we can access string object str through out your class. But why do we need to create a property here?

Can someone please help me out?

Answer 1

Properties are usually used to access instance variables from outside of the class.

For example, if you have a pointer to your class you can get/set the property like this

someObject.str = @"some string";

You also have to @synthesize the property, usually like this:

@synthesize str = _str;

In that snippet, _str is the instance variable that the property sets and gets.

The properties are essentially just shortcuts for creating accessors (setters/getters).

For example, a setter using your property would look something like this:

- (void)setStr:(NSString *)str {
    if(str != _str) {
        [_str release];
        _str = [str retain];
    }
}

and the getter would look something like this:

- (NSString *)str {
    return _str;
}

By creating properties, the previous two code blocks are automatically created for you.