Question

I notice that you can ‘double declare’ a variable in this way:

@interface A {
    NSString *instanceVariable;
}
@property (nonatomic, retain) NSString *instanceVariable;
@end

This has the same effect that just do:

@interface A {
}
@property (nonatomic, retain) NSString *instanceVariable;
@end

Why doesn’t the compiler complain in situations like this?

Answer 1

Because both ways are valid.

Declaring ivar via just declaring a property for it is a new language feature available starting objc 2.0

In “Run-time differences” section of “Objective-c programming language” reference stated:

For @synthesize to work in the legacy
runtime, you must either provide an
instance variable with the same name
and compatible type of the property or
specify another existing instance
variable in the @synthesize statement.
With the modern runtime, if you do not
provide an instance variable, the
compiler adds one for you.