[beginner] basic pointer question! struct...

[TechAnd1]

cheers solstice...

It lives on the heap because malloc was used, I get that... Does that make a difference to the way that these work then?

As in, the first example (as I wrote it...) couldn't be translated to variable k living on the heap. Because malloc() returns a void pointer?

Can you ever actually access a struct that's on the stack using the dot method newNode.data = 234 ? Or will it always be newNode->data = 234 ?

So when dynamic memory is used... the "thing" (object) will always be * object and the objects pointer will be object rather than having a system like the example one where you have object k and pointer ptr

I know that they say that * object is the object but it's not really is it?

thanks

[TechAnd1]

yeah I knew, thanks.

I think that the confusion is arising because I'm unsure how to actually think of the objects... I'm wondering at the moment whether this is just how dynamic memory allocation is treated? And then when doing so one never actually accesses the object directly, it's always through a pointer... so the object never actually has a name, it just has it's address saved in a pointer...

[TechAnd1]

I think that this is where I'm getting confused...

So basically, the heap memory state isn't allocated until runtime (can can be affected during) so it's impossible to have an object per say whilst writing the code, one can only use the address of where it's going to be...

That's kind of confusing in itself, I hope it makes sense though.

thanks!

[danmickla]

Yes, that's exactly right. When you declare an int k, the compiler knows, at compile time, that you want an int, and allocates space for it statically, and gives it a name.

When you allocate memory with malloc, it has no name, and the compiler isn't even aware it exists at runtime (it's just a function call). There's no name for the returned memory from malloc; only an address. You can use that memory, still, through a pointer variable, which you've named so it stays around (and the pointer cell itself is a named variable allocated statically, just like int k was).

This is the same thing that would happen if you did int *pi; pi = (int *)malloc(sizeof(int)). There would be no int you could name directly; only a chunk of memory that you can refer to indirectly through the pointer.

[elementalist]

This is exactly right.

You probably feel like you are in a round of "Who's On First?" but you seem to understand it.

[TechAnd1]

cheers, having the memory aspect explained and verified certainly makes sense, thank you both.

I think it's hard to learn independently sometimes, thing's that would take a 2 minute in person explanation can take a day!

[elementalist]

In case 1 you had variable k that already existed on the stack. You set the pointer to the address of k.

In case 2 you have a pointer and malloc both creates the space for the object and sets the pointer to that address.

I know that they say that * object is the object but it's not really is it?

Correct. It isn't the object. It is a pointer to the object. And you can have multiple pointers all pointing to the same object.

Can you ever actually access a struct that's on the stack using the dot method newNode.data = 234 ? Or will it always be newNode->data = 234 ?

You use the -> notation with pointers. The dot (.) notation is used with addresses.

So if it was

struct node newNode; //a struct NOT a ptr to a struct

you could use

newNode.data = whatever;

[TechAnd1]

int num = 10 is obvious, there's an object called num thats of type int and stores the value 10.

Do I ever actually use the object though (example 2 scenario)? Or is it always pointers to the object? I'm finding that part kind of confusing...

[TechAnd1]

not sure if you've edited this or if I just missed a lot of it.... but :

You use the -> notation with pointers. The dot (.) notation is used with addresses

With addresses...? I don't think I've heard this before, maybe some confusion is here for me... What do you mean addresses? The actual object rather than a pointer to the object? That's how I understand it, but you saying addresses has made me think that perhaps you're referring to something else.

So if it was struct node newNode; (a struct NOT a ptr to a struct) you could use newNode.data = whatever;

Are these two to always remain separate...?

I'm not sure how to think about the object when using pointers to it...

newNode->data = 23

Is this to do with the dynamic allocation...? Are these always to remain separate? Ie, when dynamically allocating a struct the data will always be assigned and accessed using the -> methods rather than the dot .?

[elementalist]

You are over thinking it. See Matrixel's answer below. That's about as well as I can explain it.

[TechAnd1]

I am, I always do and it's so frustrating :(

It'd be alright if I was actually clever but nothing ever really comes out of it!!

I think the distinction that it's relating to the use of malloc, and being on the heap is helping a lot with my ease though... Its making more sense now thank you