Can't understand pointers with objects

Question

It's all clear with primitives. But I'm a little bit confused with objects pointers. 
-(void) someMethod {
    NSString *originalString = @"string";
    [self changeString:originalString];
    NSLog(@"original string: %@", originalString);
}
(void)changeString:(NSString *)myString {
myString = @"new string";
NSLog(@"changed string: %@", myString);
}
The output:
changed string: new string
original string: string
I have several questions:
1) Why when we increment i we use *: *x = *x+1; But when we use NSString we don't use *: myString = @"new string"; ?
2) Why originalString hasn't changed if we passed variable by reference: (NSString *)myString ?

Dylan Shine · Answer

Alright, so in Objective-C objects are created on the Heap rather than on the Stack. Think of the Stack as containing all your primitive variables like integers. At memory level, an Objective-C object is simply a chuck of code configured in a particular way that can be created and destroyed at will. When we instantiate an Objective-C object, what were actually doing is creating a Stack object that "points" to a heap object containing your object.
int a = 1 // Stack Object
NSString *string = @"String" // Stack Object that points to a Heap object, hence the pointer
If you would like to learn more check out this article: https://mikeash.com/pyblog/friday-qa-2010-01-15-stack-and-heap-objects-in-objective-c.html
In terms of *x = *x + 1, that code shouldn't run.
You only refer to the variable as a pointer during instantiation.
Secondly, you would never increment an Objective-C object with an integer
int a = 1
a = a + 1 // Allowed
NSNumber *a = @1
a = a + 1 // Not allowed...cant add object of type NSNumber with int
I hope that helps.

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Igor Shvetsov

Igor Shvetsov

Can't understand pointers with objects

1 Answer

Dylan Shine

Dylan Shine