How do I convert NSMutableArray to NSArray?

Question

How do I convert NSMutableArray to NSArray in objective-c?

Answer 1

NSArray *array = [mutableArray copy];

Copy makes immutable copies. This is quite useful because Apple can make various optimizations. For example sending copy to a immutable array only retains the object and returns self.

If you don't use garbage collection or ARC remember that -copy retains the object.

Answer 2

An NSMutableArray is a subclass of NSArray so you won't always need to convert but if you want to make sure that the array can't be modified you can create a NSArray either of these ways depending on whether you want it autoreleased or not:

/* Not autoreleased */
NSArray *array = [[NSArray alloc] initWithArray:mutableArray];

/* Autoreleased array */
NSArray *array = [NSArray arrayWithArray:mutableArray];

EDIT: The solution provided by Georg Schölly is a better way of doing it and a lot cleaner, especially now that we have ARC and don't even have to call autorelease.

Answer 3

I like both of the 2 main solutions:

NSArray *array = [NSArray arrayWithArray:mutableArray];

Or

NSArray *array = [mutableArray copy];

The primary difference I see in them is how they behave when mutableArray is nil:

NSMutableArray *mutableArray = nil;
NSArray *array = [NSArray arrayWithArray:mutableArray];
// array == @[] (empty array)

NSMutableArray *mutableArray = nil;
NSArray *array = [mutableArray copy];
// array == nil

Answer 4

In objective-c :

NSArray *myArray = [myMutableArray copy];

In swift :

 var arr = myMutableArray as NSArray

Answer 5

Objective-C

Below is way to convert NSMutableArray to NSArray:

//oldArray is having NSMutableArray data-type.
//Using Init with Array method.
NSArray *newArray1 = [[NSArray alloc]initWithArray:oldArray];

//Make copy of array
NSArray *newArray2 = [oldArray copy];

//Make mutablecopy of array
NSArray *newArray3 = [oldArray mutableCopy];

//Directly stored NSMutableArray to NSArray.
NSArray *newArray4 = oldArray;

Swift

In Swift 3.0 there is new data type Array. Declare Array using let keyword then it would become NSArray And if declare using var keyword then it's become NSMutableArray.

Sample code:

let newArray = oldArray as Array

Answer 6

If you're constructing an array via mutability and then want to return an immutable version, you can simply return the mutable array as an "NSArray" via inheritance.

- (NSArray *)arrayOfStrings {
    NSMutableArray *mutableArray = [NSMutableArray array];
    mutableArray[0] = @"foo";
    mutableArray[1] = @"bar";

    return mutableArray;
}

If you "trust" the caller to treat the (technically still mutable) return object as an immutable NSArray, this is a cheaper option than [mutableArray copy].

Apple concurs:

To determine whether it can change a received object, the receiver of a message must rely on the formal type of the return value. If it receives, for example, an array object typed as immutable, it should not attempt to mutate it. It is not an acceptable programming practice to determine if an object is mutable based on its class membership.

The above practice is discussed in more detail here:

Best Practice: Return mutableArray.copy or mutableArray if return type is NSArray