2 ms·

I might have learned all this too long ago and lost touch with how it's like to learn this, but: how is learning about using pointers in C different from using

by IsTom 2mo ago

I might have learned all this too long ago and lost touch with how it's like to learn this, but: how is learning about using pointers in C different from using object references and array indices in python/js/java?

mannschott 2mo ago
In C a pointer can point to any arbitrary offset within an object or array, to a local variable somewhere on the stack, to a static variable, to read-only memory, to the first(?) of an indeterminate number of characters (hopefully) terminated by ((char)0). To the first of a statically unknown number of variables located somewhere in memory, to a variable of indeterminate type (thanks to union), to I/O devices, and also to unallocated memory since there's no bounds checking. A reference can only point to an object or an array on the heap. Array indexing is bounds-checked. It seems obvious to me that these restrictions make references easier to reason about than C-style pointers, and therefore easier to learn.
- IsTom 2mo ago
  I imagine most of these examples (besides strings) isn't something you learn in the beginning and when e.g. doing operations on linked lists you're pointing to either allocated structs or null.
  - jjav 2mo ago
    You tend to learn it early not by choice, but because the student makes some operation on the pointer and suddenly all they get are core dumps or (seemingly) bizarrely corrupted data and they have to confront what's going on early in the learning process.
- kbelder 2mo ago
  >It seems obvious to me that these restrictions make references easier to reason about than C-style pointers, and therefore easier to learn. I came to C from Assembly, so maybe I'm in a different camp, but references seem to me to be more difficult to reason about than pointers. Pointers just are, like math; references seem like a bunch of semi-arbitrary conventions. Like the difference between writing SQL and using an ORM, which is supposed to be simpler but ends up being messier.
jjav 2mo ago
> how is learning about using pointers in C different from using object references and array indices in python/js/java? That's exactly what trips up people coming from higher level languages! If you think of it in terms of objects and algorithms (linked lists etc) it is basically the same thing, so it feels like the same thing until things break in ways that make no sense (to people coming from high level languages). To someone coming from assembly, you just tell them a pointer is like a register holding a memory address and everything is immediately obvious. (I came to C from assembly so that was my experience) But people coming from high level languages expect the semantics that a pointer is somehow bound to an object or data structure, which it can pretend to be, but ultimately it's just a register holding a memory address. So it can point to the first byte of the memory space holding your object, or it can point to the middle of it or to any random spot in memory. Or you can reinterpret what structure you pretend it is pointing at just by recasting. And so on.

by IsTom 2mo ago

mannschott 2mo ago

In C a pointer can point to any arbitrary offset within an object or array, to a local variable somewhere on the stack, to a static variable, to read-only memory, to the first(?) of an indeterminate number of characters (hopefully) terminated by ((char)0). To the first of a statically unknown number of variables located somewhere in memory, to a variable of indeterminate type (thanks to union), to I/O devices, and also to unallocated memory since there's no bounds checking. A reference can only point to an object or an array on the heap. Array indexing is bounds-checked. It seems obvious to me that these restrictions make references easier to reason about than C-style pointers, and therefore easier to learn.

IsTom 2mo ago
I imagine most of these examples (besides strings) isn't something you learn in the beginning and when e.g. doing operations on linked lists you're pointing to either allocated structs or null.
- jjav 2mo ago
  You tend to learn it early not by choice, but because the student makes some operation on the pointer and suddenly all they get are core dumps or (seemingly) bizarrely corrupted data and they have to confront what's going on early in the learning process.
kbelder 2mo ago
>It seems obvious to me that these restrictions make references easier to reason about than C-style pointers, and therefore easier to learn. I came to C from Assembly, so maybe I'm in a different camp, but references seem to me to be more difficult to reason about than pointers. Pointers just are, like math; references seem like a bunch of semi-arbitrary conventions. Like the difference between writing SQL and using an ORM, which is supposed to be simpler but ends up being messier.

jjav 2mo ago

> how is learning about using pointers in C different from using object references and array indices in python/js/java? That's exactly what trips up people coming from higher level languages! If you think of it in terms of objects and algorithms (linked lists etc) it is basically the same thing, so it feels like the same thing until things break in ways that make no sense (to people coming from high level languages). To someone coming from assembly, you just tell them a pointer is like a register holding a memory address and everything is immediately obvious. (I came to C from assembly so that was my experience) But people coming from high level languages expect the semantics that a pointer is somehow bound to an object or data structure, which it can pretend to be, but ultimately it's just a register holding a memory address. So it can point to the first byte of the memory space holding your object, or it can point to the middle of it or to any random spot in memory. Or you can reinterpret what structure you pretend it is pointing at just by recasting. And so on.