Does NSMutableArray actually count the items every time its count method is called?

Question

Because of cocoa design patterns, the name of the instance method count in NSMutableArray is ambiguous; it could either return a saved variable

Answer 1

NSArray declares a private ivar _used that appears to hold the count of the array. Using object_getInstanceVariable() you can verify that in the case of the concrete subclass __NSArrayM, this variable is incremented and decremented as objects are added and removed.

I used the following program, a Mac OS X Foundation command-line tool, to verify this:

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

int main(int argc, const char * argv[])
{

    @autoreleasepool {

        NSMutableArray *anArray = [NSMutableArray array];
        NSLog(@"anArray's concrete class is %@", [anArray class]);
        UInt64 used = 0;
        for (NSString *toAdd in @[@"o", @"m", @"g"]) {
            object_getInstanceVariable(anArray, "_used", (void **)&used);
            NSLog(@"array = %@, used = %lld", anArray, used);
            [anArray addObject:toAdd];
        }
        for (NSString *toRemove in [anArray copy]) {
            object_getInstanceVariable(anArray, "_used", (void **)&used);
            NSLog(@"array = %@, used = %lld", anArray, used);
            [anArray removeObject:toRemove];
        }
        object_getInstanceVariable(anArray, "_used", (void **)&used);
        NSLog(@"array = %@, used = %lld", anArray, used);
    }
    return 0;
}

This program produced the following output:

2013-01-31 17:40:15.376 Array[10173:303] anArray's concrete class is __NSArrayM
2013-01-31 17:40:15.378 Array[10173:303] array = (
), used = 0
2013-01-31 17:40:15.378 Array[10173:303] array = (
    o
), used = 1
2013-01-31 17:40:15.379 Array[10173:303] array = (
    o,
    m
), used = 2
2013-01-31 17:40:15.379 Array[10173:303] array = (
    o,
    m,
    g
), used = 3
2013-01-31 17:40:15.380 Array[10173:303] array = (
    m,
    g
), used = 2
2013-01-31 17:40:15.380 Array[10173:303] array = (
    g
), used = 1
2013-01-31 17:40:15.380 Array[10173:303] array = (
), used = 0

Answer 2

As you have correctly noted, there is no guarantee that it will behave one way or the other.

In practice, though, -[NSArray count] is a constant-time operation. You can confirm this yourself by creating a small array and a large array and benchmarking the time it takes to get their counts. It would be pretty silly to have an O(n) count method on the core array classes like this.