// create an immutable array
NSArray *arr = [NSArray arrayWithObjects: @"one", @"two", @"three", nil ];
// create a mutable copy, and mutate it
NSMutableArray *mut = [arr mutableCopy];
[mut removeObject: @"one"];
NSMutableArray *mut1 = [mut mutableCopy];
[mut1 removeObjectAtIndex:0];
NSMutableArray *mut2 = [NSMutableArray arrayWithObjects:@"A", @"B", nil];
NSMutableArray *mut2_immut = [mut2 copy];
//crash here
//[myArray_immut removeObjectAtIndex:0];
// Copys object, result is mutable
NSMutableArray *mut2_mut = [mut2 mutableCopy];
[mut2_mut removeObjectAtIndex:0];
mutableCopy和copy的区别
原文：http://blog.csdn.net/zh_2608/article/details/42001699
