Is -[NSUserDefaults registerDefaults:] additive and does it overwrite values?

Question

In other words, must I call registerDefaults in only one location in my app (e.g. when it starts up), or can I register defaults on an as-needed basis?

If it is additive, what happens when I try overwriting a value?

For example, given the following code...

[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"a": @1}];
[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"b": @2}];
[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"b": @3}];

... what will the final defaults dictionary be?

1. @{@"b": @3} (replacive)
2. @{@"a": @1, @"b": @3} (additive, overwrite values)
3. @{@"a": @1, @"b": @2} (additive, don't overwrite values)
4. Crash, since @"b" is already defined.

Answer 1

2. It is additive, and overwrites values.

Example:

[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"a": @1}];
[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"b": @2}];
[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"b": @3}];
NSAssert([[NSUserDefaults standardUserDefaults] integerForKey:@"a"] == 1, @"");
NSAssert([[NSUserDefaults standardUserDefaults] integerForKey:@"b"] == 3, @"");

The ultimate dictionary is:

@{@"a": @1, @"b": @3}

Thus, you can call it as many times as necessary.

From the documentation:

Adds the contents of the specified dictionary to the registration domain.