What are the barriers to understanding pointers an

Question

Why are pointers such a leading factor of confusion for many new, and even old, college level students in C or C++? Are there any tools or thought processes that helped you understand how pointers work at the variable, function, and beyond level?

What are some good practice things that can be done to bring somebody to the level of, "Ah-hah, I got it," without getting them bogged down in the overall concept? Basically, drill like scenarios.

Answer 1

The reason I had a hard time understanding pointers, at first, is that many explanations include a lot of crap about passing by reference. All this does is confuse the issue. When you use a pointer parameter, you're still passing by value; but the value happens to be an address rather than, say, an int.

Someone else has already linked to this tutorial, but I can highlight the moment when I began to understand pointers:

A Tutorial on Pointers and Arrays in C: Chapter 3 - Pointers and Strings

int puts(const char *s);

For the moment, ignore the const. The parameter passed to puts() is a pointer, that is the value of a pointer (since all parameters in C are passed by value), and the value of a pointer is the address to which it points, or, simply, an address. Thus when we write puts(strA); as we have seen, we are passing the address of strA[0].

The moment I read these words, the clouds parted and a beam of sunlight enveloped me with pointer understanding.

Even if you're a VB .NET or C# developer (as I am) and never use unsafe code, it's still worth understanding how pointers work, or you won't understand how object references work. Then you'll have the common-but-mistaken notion that passing an object reference to a method copies the object.

Answer 2

An analogy I've found helpful for explaining pointers is hyperlinks. Most people can understand that a link on a web page 'points' to another page on the internet, and if you can copy & paste that hyperlink then they will both point to the same original web page. If you go and edit that original page, then follow either of those links (pointers) you'll get that new updated page.

Answer 3

I don't think that pointers themselves are confusing. Most people can understand the concept. Now how many pointers can you think about or how many levels of indirection are you comfortable with. It doesn't take too many to put people over the edge. The fact that they can be changed accidently by bugs in your program can also make them very difficult to debug when things go wrong in your code.

Answer 4

Every C/C++ beginner has the same problem and that problem occurs not because "pointers are hard to learn" but "who and how it is explained". Some learners gather it verbally some visually and the best way of explaining it is to use "train" example (suits for verbal and visual example).

Where "locomotive" is a pointer which can not hold anything and "wagon" is what "locomotive" tries pull (or point to). After, you can classify the "wagon" itself, can it hold animals,plants or people (or a mix of them).

Answer 5

The way I liked to explain it was in terms of arrays and indexes - people might not be familiar with pointers, but they generally know what an index is.

So I say imagine that the RAM is an array (and you have only 10-bytes of RAM):

unsigned char RAM[10] = { 10, 14, 4, 3, 2, 1, 20, 19, 50, 9 };

Then a pointer to a variable is really just the index of (the first byte of) that variable in the RAM.

So if you have a pointer/index unsigned char index = 2, then the value is obviously the third element, or the number 4. A pointer to a pointer is where you take that number and use it as an index itself, like RAM[RAM[index]].

I would draw an array on a list of paper, and just use it to show things like many pointers pointing to the same memory, pointer arithmetic, pointer to pointer, and so on.

Answer 6

I like the house address analogy, but I've always thought of the address being to the mailbox itself. This way you can visualize the concept of dereferencing the pointer (opening the mailbox).

For instance following a linked list: 1) start with your paper with the address 2) Go to the address on the paper 3) Open the mailbox to find a new piece of paper with the next address on it

In a linear linked list, the last mailbox has nothing in it (end of the list). In a circular linked list, the last mailbox has the address of the first mailbox in it.

Note that step 3 is where the dereference occurs and where you'll crash or go wrong when the address is invalid. Assuming you could walk up to the mailbox of an invalid address, imagine that there's a black hole or something in there that turns the world inside out :)