Objective c 'this' equivalent for fields with the same name as method parameters

Question

I would really like to do something I could do in Java but can't find out how to do in objective-c with regards to naming variables. In java:

private int var;

public void aMethod(int var)
{
    this.var = var;
}

I'd really like to use the same variable name for a method parameter and a field in some cases. Is that possible? Thanks...

Answer 1

Consider this header file:

@interface MYClass : NSObject {
@private
   int var;                            // 1
}

@property(nonatomic, assign) int var;  // 2

-(void)someMethodWithVar:(int)var;     // 3

@end

In this case you have three instances of the symbol var:

1. This is a private instance variable, also called ivar.
2. This is a public property.
3. This is an argument to a method.

Since all three instances are different things the var symbol can be freely reused, they do not clash. However when implementing someMethodWithVar: you can get into trouble.

You could assign the property:

-(void)someMethodWithVar:(int)var {
    self.var = var;
}

This will work, but will give you warning "local variable hides the instance variable". It will compile and work as expected. The local variable will have the highest presidence, but the local ivar is never address, so no problem.

You could try to assign the ivar directly:

-(void)someMethodWithVar:(int)var {
    var = var;
}

Still the same warning but this time it is fatal! What you do is assigning the local variable to itself, a no-op.

You could try to assign the ivar by reference to self:

-(void)someMethodWithVar:(int)var {
    self->var = var;
}

Still a warning, but not fatal since you explicitly dereference the ivar through self.

Now warnings are BAD, you should treat warnings as error, especially if you are new and not 100% sure of what you do.

There are two ways to remove the warning. The first way, that I use myself, is to always name instance variables with a _ character prefix. This way they never clash with anything, and it is very explicit what they are.

@interface MYClass : NSObject {
@private
   int _var;
}
...

The second way is to change the argument name. Many of Apple's own methods historically do this by using a or an as a prefix.

-(void)someMethodWithVar:(int)aVar;