Difference between declaring an ivar in @interface and putting variable in @implementation

Question

What is the difference between declaring an ivar within an @interface versus putting a variable within an @implementation in a .m file?

Answer 1

As far as I know, putting a variable declaration inside the @implementation is no different from putting it outside the implementation: It's not an ivar, it's just a variable declared at file scope.

One use is for declaring the equivalent of C++ static members. For example:

@implementation MyClass

static int s_count = 0;

- (id)init {
  if ((self = [super init]))
    ++s_count;
  return self;
}

- (void)dealloc {
  --s_count;
  [super dealloc];
}

Assuming init is your only initializer, then s_count will contain the total number of instances of MyClass that are active.