Question

That’s an issue I still don’t understand.

Sometimes I have to write:

NSString* myVariable; myVariable = @'Hey!'; 

Then, for example I define a Structure ‘DemoStruct’ and get an Variable that uses it. Lets say I have a Structure that has x and y vars from type double.

I want to pass this var to a method which then manipulates my var, and I want that this manipulation has effect on the context from which I passed the var to that method. So I need a pointer, right.

I pass it to the method like that:

[someObject someMethod:&myVarThatUsesTheStruct] 

that method now looks like that:

- (void)someMethod:(DemoStruct*)myVar { (*myVar).x += 10; } 

Before the call, the component x of the struct was lets say 1000. Now, 10 is added and it is 1010 after the method call.

But I really really hardly dont get it why I have to use the Asterisk * for myVar in the Method, since I say already in the Method Header that myVar is a POINTER to a DemoStruct. I just pass with &myVarThatUsesTheStruct the memory address.

Can someone explain why this is like it is?

Answer 1

As you say, myVar is a pointer. As such, myVar.x is not correct: it would by a field of a pointer, which has no sense in C/Objective-C.

If you want to access to the variable pointed to by a pointer, you have to add the asterisk: myVar is a pointer, *myVar is the variable pointed to by myVar.

Moreover, in your case, you can use a special construct of C by writing myVar->x, which is strictly equivalent to (*myVar).x.

All of this is standard C, not specific to Objective-C.

About your first example, you don’t have to put an asterisk because you change the value of the pointer, not the value of the variable: myVariable is a pointer to an object which at declaration time is assigned the nil value. The next instruction (myVariable = @'Hey!') is an assignment of pointer values: @'Hey!' is a pointer to a NSString object. The value of this pointer (not the value of the pointed constant) is assigned to myVariable, which then points to the object @'Hey!'.

Yes, this is diffucult to follow at first time…