> It's just using "fat pointer" to refer to the concept of passing around a pointer with extra information concerning the data it points to.
Is it actually? Here's a quote: "The trick is to place the value representing the number of items in the array in memory just before the pointer we actually pass to functions. This pointer is fully compatible with normal pointers,"
So suppose I pass this "fully compatible with normal pointers" pointer to a function.. and it ends up being stored in a register. Now where is this location "just before the pointer"? In the previous register?
I don't think this post is describing fat pointers at all. I think it is describing arrays with a prepended length header. There is no data in or before the pointer, there is metadata before the pointee assuming you're pointing at something with metadata prepended to it.. No extra information is passed with a pointer, it's just assumed to be there at the pointee. Call it a fat pointee if you want a fancy name; at least that acknowledges the onus is on the pointee to have the right metadata in the right place. There's nothing in the pointer here.
Yes, the extra data being the length that is located in memory before the pointer.
> So suppose I pass this "fully compatible with normal pointers" pointer to a function.. and it ends up being stored in a register. Now where is this location "just before the pointer"? In the previous register?
You’re at the wrong level of indirection; the header in in the memory before what the pointer points to, as in p - 1 rather &p - 1.
> I think it is describing arrays with a prepended length header.
Correct.
> Call it a fat pointee if you want a fancy name; at least that acknowledges the onus is on the pointee to have the right metadata in the right place.
I would prefer that this was not called a “fat pointer”, but the claim made above is that a pointer with any implicit data associated with it is “fat”.
> There's nothing in the pointer here.
No data, but there is an implicit guarantee that it points to the data portion of a length-prepended array.
> Yes, the extra data being the length that is located in memory before the pointer.
> You’re at the wrong level of indirection; the header in in the memory before what the pointer points to, as in p - 1 rather &p - 1.
You are contradicting yourself. &p - 1 is before the pointer. p - 1 is before the pointee.
> I would prefer that this was not called a “fat pointer”, but the claim made above is that a pointer with any implicit data associated with it is “fat”.
> No data, but there is an implicit guarantee that it points to the data portion of a length-prepended array.
I think that's a borderline useless definition. In my C, there's an implicit guarantee that any pointer, in a context where it may be dereferenced, points to a valid object (as long as it's not NULL and there's no programming error causing it to point at nothing well defined). Usually it points at the start of a struct, sometimes it points at list node (or whatever) embedded in a struct and I might have to work my way back with offsetof. Either virtually all of my pointers are fat or none of them are. I go by that none of them are, because whatever data I have (implicit or explicit) is not a property of the pointers I use but of the data I point them to.
Is it actually? Here's a quote: "The trick is to place the value representing the number of items in the array in memory just before the pointer we actually pass to functions. This pointer is fully compatible with normal pointers,"
So suppose I pass this "fully compatible with normal pointers" pointer to a function.. and it ends up being stored in a register. Now where is this location "just before the pointer"? In the previous register?
I don't think this post is describing fat pointers at all. I think it is describing arrays with a prepended length header. There is no data in or before the pointer, there is metadata before the pointee assuming you're pointing at something with metadata prepended to it.. No extra information is passed with a pointer, it's just assumed to be there at the pointee. Call it a fat pointee if you want a fancy name; at least that acknowledges the onus is on the pointee to have the right metadata in the right place. There's nothing in the pointer here.