Objective-C: Where is the pointer that corresponds to an instance variable of an object stored

Question

Suppose I have the following class definition

@interface ClassX: NSObject
@property NSArray *arr;
@end

Suppose I have the following lines in a method

-(void)someMethod
{
  ClassX *obj = [ClassX new];
  obj.arr = [NSArray arrayWithObjects:[NSNumber numberWithInt:2], [NSNumber numberWithInt:3], nil]; //Edit to avoid getting derailed by differences due to @ notation
}

What is in the stack, what is in the heap?

Stack:

obj in the stack. Value of this variable is the address of the place in the heap where obj resides.

Heap:

Object of type ClassX

What about the NSArray and the NSNUmbers within the NSArray? Are they also divided as above? I.e The pointer to the NSArray is in the stack and the NSArray object is in the heap? If yes, what is contained in the heap memory that contains the "obj" object?

The figures I have seen are that this block of memory in the Heap contains a isa pointer and then instance variables. The isa pointer point to the location of the Class structure. I.e this piece of memory contains another isa pointer followed by Method structs. Each struct has a selector and a pointer to where the corresponding implementation starts.

This seems to suggest that the area variable within the object is in the heap. I.e the pointer to the NSArray object is in the heap.

EDIT Based on answers below, is this a correct view of what is there in heap and stack?

The stack will contain 4 pointers, One to obj, one to NSArray, two to NSNumber objects.

The heap memory that starts at the address pointed to by obj has 1. the isapointer pointing ClassX class structure 2. Followed by memory to store NSArray object.

This NSArray object memory has 1. isapointer pointing to NSArray class structure 2. followed by memory to store 2 NSNumber objects.

The NSNUmber object memory contain 1. A isapointer pointing to NSNumber class structure 2. Followed by memory to store 2 ints

You have a constant array with constant contents, so the array and numbers will be in the data segment of your app. obj.arr will contain a reference to the static array in this data section. obj will be in the heap. — Paulw11
@Paulw11 That's what happens with regular arrays, but it is not what happens to @-initialized arrays and @ constants. — Sergey Kalinichenko
@Paulw11, what do you mean by data segment? Is this on the heap or on the stack? What if I the array is a NSMUtableArray instead of NSArray? My main question is what is in the stack and what is the content of the memory in the heap. I have put my understanding based on current answers under Edit in the question above. Can you take a look? — Smart Home
The data segment is neither heap nor stack; it's a region of memory whose (initial) values are copied directly from your program's binary. — jscs

Sergey Kalinichenko Sergey Kalinichenko · Accepted Answer · 2016-10-08T23:03:36

Both NSArray and NSNumber objects are heap-allocated, not stack-allocated.

They do not look like heap objects because the compiler hides it from you using the relatively new syntax @[].

When you write

@[@1, @2]

The compiler translates it to

[NSArray arrayWithObjects: {[NSNumber numberWithInt:1], [NSNumber numberWithInt:2]} count: 2]

The end result is as follows: the stack has a pointer to an object of type ClassX. The memory pointed to by this pointer starts with an isapointer, followed by a pointer to another location in the heap that has NSArrray object in it. In turn, this part of heap memory has another isapointer followed by a pointer to a block of memory representing the inner array of NSArray, containing two pointers to NSNumber objects. Each of the NSNumber objects has an isapointer followed by data which represents the number.

Objective-C: Where is the pointer that corresponds to an instance variable of an object stored

1 Answers