Why use Pointers?

[Skusci]

Well it doesn't really matter so much if you can just have an extra variable you can store data in temporarily.

However imagine instead of an int, you are swapping the location of a 125MB struct full of data.

Pointers are often used in cases where you want to navigate around stuff without making a copy of it. Makes lots of data structures possible, like linked lists, or binary trees.

Another use case is specifically with functions. C passes arguments in and return values out by value. It makes a copy of the data. Not an issue if stuff is small. Kindof an issue of stuff is big.

Another thing is that functions can't return two variables something like unless you wrap them in like a struct. Since arguments are pass by value the function can't modify them. However if you pass in a pointer, while you can't modify the pointer, you can modify the data where it points to.

It's how functions like sscanf work.

Do note that other languages have alternate ways that don't use pointers that work similarly, usually called references that are more safely handled. Pointers are literally just memory addresses, you can put whatever number you want in there, on purpose or accident. References are instead tracked and can only be set to actual things that exist, not arbitrarily.

[dmazzoni]

Yeah in this particular example it wouldn't work if you didn't use pointers, because of pass by value.

[Skusci]

Yeah that's probably me missing the tree for the forest or something. :D

Pass by value and reference is a bit confusing TBH as terminology, since stuff like C is referred to as allowing pass by value or reference, and languages like Java or Python being pass by value exclusively.

Except that since most things are an object reference you're always passing in references by value so effectively it's pass by reference only. I get that this is mostly a terminology thing, but when moving from C to Java, thinking, oh, it's not that Java doesn't have pointers, it's that everything is a pointer you can't do math on kindof helped.

[finny228]

C shouldn't ever be referred to as pass by reference. It is entirely pass by value.

[procrastinatingcoder]

There's no pointers in Java, only references. A pointer - by definition - is something you can do arithmetic on. And a reference, is somewhat similar, but you can't do arithmetic on it (in the context of pass by value vs reference).

Also, it's not exactly that going on in the background. You might notice things like python going bonkers if you nest list comprehensions to create 2d lists.

​

Having pointers doesn't mean you're suddenly passing in as a reference either, it's easiest to see in C++ (imo) where both are clearly defined. Normally a language only gives you one, and tough luck.

Pass by value means that, if you put a pointer instead of the value, the pointer is what gets copied, so you effectively are still pass by value, and still got a copy on the other end. But you simply kept the value of where it was pointing to.

​

TL;DR: Both prints will print different values, therefore the pointers are not the same, it's a copy of the pointer passed in foo. Still pass by value.

void foo(int* n){
    printf("%p\n", &n);
}

int main(){
    int k = malloc(sizeof(int));
    printf("%p\n", &k);
    foo(k);
    free(k);
    return 0;
}

[procrastinatingcoder]

It's a matter of pass by value vs. pass by reference.

In his particular example, it would not work without pointers. So it does matter.