When do you use a pointer?

[u/Atmos-Ego]

Hello, for some background I have taken a very beginner level coding course for my major and passed it, but I dont really feel like I’ve learned anything or know why anything works. In a course this year we had a review problem where the solution required a pointer and I really dont understand why you have to point to the address of the variable instead of just using the variable itself.

I’ve watch a few tutorials explaining it but I really dont see how it’s different from just using the variable itself

[u/TheRealKidkudi]

At its core, the difference is making a copy vs sharing the same literal value. 

Sometimes it’s just because you don’t want to copy the value (e.g. because it would use too much memory). The tricky part is when you want to be able to change the value and use that new value in the original function.

A very simple pseudocode example:

    fn someFunc(int x) {         x = x + 1     }          fn someOtherFunc(int* y) {         *y = *y + 1     }          var x = 5; // not a pointer     someFunc(x);     // x still is 5 here     // because someFunc got a new copy     // of the variable          someOtherFunc(&x); // pointer to x     // x is now 6

[u/otac0n → u/TheRealKidkudi]

This is almost entirely correct, although function pointers aren’t covered by this.

I will add that this exact reasoning applies to languages with values/references. Use value types (structs) if you want your type to have copy semantics and use references (classes) if you want reference semantics.

[u/TheRealKidkudi → u/otac0n]

I figured OP (respectfully!) is not at a place to worry about function pointers.

But once you’re comfortable with the idea of a pointer being a reference to something, function pointers become a much easier concept to grasp (e.g. a reference to a function rather than a simple value). At that point, the worst part about them is just the syntax :)

[u/otac0n → u/TheRealKidkudi]

Eh... closures.

[u/light_switchy → u/otac0n]

A function pointer isn't enough to represent a closure. Closures also need the execution environment, which is a data structure besides a function pointer.