What is the real difference between Pointers and References?

Question

AKA - What\'s this obsession with pointers?

Having only really used modern, object oriented languages like ActionScript, Java and C#, I don\'t really understand the

Answer 1

Strings are fundamental to C (and other related languages). When programming in C, you must manage your memory. You don't just say "okay, I'll need a bunch of strings"; you need to think about the data structure. How much memory do you need? When will you allocate it? When will you free it? Let's say you want 10 strings, each with no more than 80 characters.

Okay, each string is an array of characters (81 characters - you mustn't forget the null or you'll be sorry!) and then each string is itself in an array. The final result will be a multidimensional array something like

char dict[10][81];

Note, incidentally, that dict isn't a "string" or an "array", or a "char". It's a pointer. When you try to print one of those strings, all you're doing is passing the address of a single character; C assumes that if it just starts printing characters it will eventually hit a null. And it assumes that if you are at the start of one string, and you jump forward 81 bytes, you'll be at the start of the next string. And, in fact taking your pointer and adding 81 bytes to it is the only possible way to jump to the next string.

So, why are pointers important? Because you can't do anything without them. You can't even do something simple like print out a bunch of strings; you certainly can't do anything interesting like implement linked lists, or hashes, or queues, or trees, or a file system, or some memory management code, or a kernel or...whatever. You NEED to understand them because C just hands you a block of memory and let's you do the rest, and doing anything with a block of raw memory requires pointers.

Also many people suggest that the ability to understand pointers correlates highly with programming skill. Joel has made this argument, among others. For example

Now, I freely admit that programming with pointers is not needed in 90% of the code written today, and in fact, it's downright dangerous in production code. OK. That's fine. And functional programming is just not used much in practice. Agreed.

But it's still important for some of the most exciting programming jobs. Without pointers, for example, you'd never be able to work on the Linux kernel. You can't understand a line of code in Linux, or, indeed, any operating system, without really understanding pointers.

From here. Excellent article.

Answer 2

Historically, what made programming possible was the realization that memory locations could hold computer instructions, not just data.

Pointers arose from the realization that memory locations could also hold the address of other memory locations, thus giving us indirection. Without pointers (at a low level) most complicated data structures would be impossible. No linked-lists, binary-trees or hash-tables. No pass by reference, only by value. Since pointers can point to code, without them we would also have no virtual functions or function look up tables.

Answer 3

Pointers are the most pragmatic way of representing indirection in lower-level programming languages.

Answer 4

Parameter efficency - passing a pointer (Int - 4 bytes) as opposed to copying a whole (arbitarily large) object.

Java classes are passed via reference (basically a pointer) also btw, its just that in java that's hidden from the programmer.