Question

I would like to pass a NSMutableArray by reference so that it can be altered by another method. What would be the correct syntax for this?

Thanks,

Answer 1

Objective-C objects are always passed by reference (using pointers) – you can’t pass them by value.

I.e. the following is fine:

- (void)mutateArray:(NSMutableArray*)array {
    // alter array ...
}

… and can be e.g. invoked like this:

NSMutableArray *array = ...;
[self mutateArray:array];

There is also the possibility of passing a pointer by reference:

- (void)newArray:(NSMutableArray **)array;

In that case array is used as an out-parameter – you pass a reference to a pointer to receive an instance:

- (void)newArray:(NSMutableArray **)array {
    *array = [[NSMutableArray alloc] init];
}

… which could be called like so:

NSMutableArray *array = nil;
[self newArray:&array];

Using out-parameters is usually only seen if the return-value is already used and additional information has to be returned. An example would be error-information as dreamlax noted.