Passing An Array By Reference In Objective-C

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.

Answer 2

In addition to Georg Fritzche's answer, it may be worth noting that some methods expect to be given the address of an object pointer. For example:

NSError *anError; // points to garbage now
NSStringEncoding enc;
NSString *aString = [NSString stringWithContentsOfFile:@"/some/file.txt"
                                          usedEncoding:&enc
                                                 error:&anError];

if (aString == nil)
{
    // anError now points to an initialised NSError object.
}

It gets tricky because some documented methods require you to release objects obtained in this manner, and some don't (for an example of one that does require explicit releasing, see NSPropertyListSerialization).