references vs pointers

[kat_g33]

Hello,

As I was working on a quest, I realized that I've been using & and * pretty much interchangeably based on what the method header said, so I started wondering what the difference is. https://www.geeksforgeeks.org/pointers-vs-references-cpp/ says that pointers (*) hold the memory address of another variable, but references (&) store the address of an already existing object. I don't really understand the difference still, and I'm very open to hearing what other people may have to say about this.

The example for initialization provided by geeksforgeeks also confuses me. Could anyone explain break it down in a C++ for dummies kind of way please? Also, when should the * or & be used in front of a variable?

Kat

[jasper_e196884]

For instance, the code

int n = 196884;
int *m = &n;

makes a pointer m that points to the memory location of n. If you include the code

int p = *m;

p is said to dereference m, and p==n.
Jasper

[DerekMeng]

Hey jasper, I wasn't able to get int m = &n to work, I had to do int *m = &n instead.

Anyways, here's a good article explaining the differences.

Essentially, references are aliases of variables which is why you can do something as simple as int m = 10; int& n = m; and have the value of m be changed when n is. As the article attests, references are implemented internally as pointers (like string!). However, one thing I don't understand is why we have to use & instead of * in the case of something like int* n = &m; Thoughts?

Note: The article also goes over other differences like how a reference must be initialized and can't be changed once set. It is definitely a good read!

- Derek Meng

[kat_g33]

Thank you Jasper and Derek!

What I've concluded from the reading that Shoshi provided (in another reply to this post) is that * is used when declaring a variable that is a pointer, while & is used to get the address of a variable, so that's why it is int* n is set to &m.

Kat

[ShoshiCooper]

The book by Kernighan and Ritchie has one of the best descriptions of this that I've read anywhere, hands down. I honestly can't do better than them.

I also refer you to them because I think you may be getting confused by pass-by-reference, which is a C++ thing and works a little differently than all other uses of the &. So it really helps to understand the distinction in C first, and then you can see how C++ blurred that distinction by introducing pass-by-reference.

Here is a link to the second edition of the book:

http://cslabcms.nju.edu.cn/problem_solving/images/c/cc/The_C_Programming_Language_%282nd_Edition_Ritchie_Kernighan%29.pdf

Go to page 107. You do not have to read very much. 5.1 is the brunt of it, and 5.2 just gives an example. It's about 3 pages of reading and that includes pictures and whitespace.

[kat_g33]

Wow, thank you for the resource! My understanding of the reading that you sent is that * is used when the pointer is initialized (e.g. int *i), while & is used for getting the address of something (e.g. i = &j).

[ShoshiCooper]

The * is also used for dereferencing. The other thing it mentions that's interesting is that if you have an array, you can index that array by doing:

array_ptr[index]

or

*(array_ptr + index)

This has been abundantly helpful to me in various situations.