Edit: Perhaps I mean “how” are they different from references, not why.
In: Technology
This is really, really tough to distill down to ELI5. In particular, the difference between a pointer and a reference takes some pretty out-there thought. But let’s go.
Think about a house. A house is like a variable. It is where it is, it is what it is. If I say I’m giving you that house, then you get that house. If we both want that house, one of us has to build another house just like it so there are 2 houses. This is how “variables” work.
That house can have an address. Let’s say it’s 123 Main Street. Every house has an address, that’s how we tell people which house we mean. This isn’t quite pointers yet, but it’s important to know that houses have addresses, and variables have addresses. You can’t really *change* the house’s address, though. (Nitpicky aside: your city might rename or renumber the street, but this is really rare, so we’re going to pretend it doesn’t happen, OK?) If you move, the house at 123 Main Street is still the house at 123 Main Street. You’re in a different house with a different address now. This reflects that, with variables, knowing that they have an address doesn’t really help us do much.
We call addresses, when used in this way, “indirection”. That is to say, once I have pieces of paper and addresses, I can talk about a house without having to be close enough to point. If I give you the address and say, “Here’s the house I mean”, you know the paper isn’t a house. You understand it is instructions for how to find a specific house. Indirection.
Now. Imagine I write “123 Main Street” on a piece of paper. NOW I have a pointer. I can give you the piece of paper, and you can find the house. The paper “points to” the house. When you get there, maybe you want to give someone else directions. You can erase what I wrote and write a different address on the paper. So that piece of paper can point at any house in the world! All you have to do is write a new address on it.
So. References. Maybe I gave you that piece of paper to give to someone else. And maybe I wanted them to see “123 Main Street” and I’m mad that you wrote another address on it. So next time, I give you a stone tablet into which I carved “123 Main Street”. Now you can’t erase the address and replace it with a different one. (Not easily, at least.) This is how references are different from pointers. They still provide indirection, but it’s a kind that can’t be changed so easily.
There are some other subtle differences. For example, technically a piece of paper can refer to ANYTHING. Suppose I have 10 pieces of paper with 10 different house addresses on them. Now suppose I write the numbers 1-10 on them. And I give you a post-it that says “Address number 3”. So you pick up the other paper that has “3” on it, and go to the house at that address. My post-it has the address of an address on it now! This is a pointer-to-a-pointer, or “a second level of indirection”. You cannot do this with references.
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