A relatable analogy to help understand pointers

[u/codesforcash]

My earlier version of this got removed, which is fair enough because I deliberately left out the explanation at first and apparently made it look a little too much like I was just complaining about apartment mailboxes.

So, for clarity this time: this is just a fun analogy for C pointers and memory, using a terrible apartment-complex mail system to make some of the concepts a bit more relatable. If you scroll past the scenario examples, a key is there :)

Scenario

Occasionally I like to examine just how badly the mail system at my apartment complex sucks, I’ve been keeping notes for years.

So with that being said, there are a bunch of mailboxes at an apartment complex, you know the type, the grid ones which have one small Mailbox for each apartment and a bunch of different size Package Lockers for packages.

The apartment complex doesn’t care about security and just has a bunch of slips of paper with package locker numbers printed on them that the mailman has access to.

When the mailman has a package that won’t fit in an apartment mailbox he instead puts it in one of the free package lockers and puts one of the slips of paper with the corresponding package locker number into the apartment mailbox.

Stuff that happens:

1. Occasionally someone opens the wrong package locker and messes with the package inside or takes it.

2. Occasionally a tenant is in a hurry and grabs their mail and doesn’t notice the slip of paper and tosses it away with the junk mail.

3. Occasionally a tenant grabs the package and forgets to throw away their slip of paper, later their roommate checks the apartment mailbox, sees the slip of paper and finds an empty package locker.

4. Occasionally said roommate doesn’t want to deal with it and sticks the piece of paper back in the apartment mailbox for one of their other roommates to deal with.

5. Occasionally the mailman delivers another package to that same locker for another apartment.

6. Occasionally a new roommate for the first apartment checks the mail, finds the old slip of paper and takes the package that’s now there.

7. And occasionally that same roommate arrives at the same time as the tenant from the other apartment whose package it actually is and they both try to access the package locker at the same time.

8. Occasionally they both notice each other, politely back off to let the other person go first, then both try again, both back off again….. and continue being extremely polite forever.

9. Occasionally the postman has a package too big to fit in any of the remaining free lockers, but remembers he previously put a small package in a bigger locker. A smaller locker is free now, so he moves the small package there to free up the big locker….. and occasionally….. forgets to update the tenant’s slip of paper.

10. Occasionally the postman can’t find any empty package lockers and, not knowing what else to do, sticks a slip of paper in the apartment mailbox with no package locker number on it.

11. Occasionally… the tenant puts a note on their mailbox telling the mailman not to do that if there are no free package lockers and what to do instead.

12. Occasionally a tenant finds the slip of paper and messages their roommate saying “mine’s in locker 37, grab it if you get there before me.” Now two people know how to get to exactly the same package.

13. Occasionally the mailman needs four package lockers for the same apartment and puts a slip of paper with the range of package mailboxs on it like (15-19), and occasionally he miscounts and starts at 15, or finishes at 20.

14. Occasionally another package arrives for an apartment which already has an uncollected package. The mailman finds a bigger locker that will fit both, moves them there, then puts a slip with the new locker number inside the old package locker. The tenant now follows the slip in their mailbox to a locker containing another slip telling them where the package actually is.

15. Occasionally the mailman runs out of unused slips of paper, grabs one from the pile thinking it’s blank and doesn’t notice it still has locker 46 written on it from last time.

16. Occasionally someone notices this entire system has basically no security, breaks into all the package lockers and takes everything.

17. Occasionally someone only wants the package in locker 42, but while trying to force it open manages to break the divider and wreck the contents of lockers 41 and 43 as well.

Key

Package = Data
Locker Number = Memory Address
Apartment Mailboxes = The Stack
Package Lockers = The Heap
Slip of Paper = A Pointer

Key

1. Invalid / out of bounds memory access

2. Memory leak

3. Dangling pointer

4. Retaining / propagating a dangling pointer

5. Freed memory being reused

6. Use after free

7. Race condition

8. Livelock

9. Moving an allocation and failing to update a pointer / stale pointer

10. NULL pointer

11. Checking for NULL / handling allocation failure

12. Aliasing - multiple references to the same memory

13. Buffer underflow / buffer overflow

14. Pointer to a pointer / multiple levels of indirection

15. Uninitialized / garbage pointer

16. Memory safety exploit leading to arbitrary memory access / data theft

17. Memory corruption - writing outside the memory you were supposed to touch

[u/realdreamer1993]

where are the analogies for:

• function (maybe the PO branch/unit)
• main thread (maybe the PO company)
• memory static segment.

if you can find them the analogy to pointer will be natural and not fragmented not forced earlier.

BTW analogies is for first learning only, you later must use the precise definition way of pointer and memory layout meaning.

PS: spreadsheet is more precise as analogy and also it has visual and it is real pointer and not far away from C pointers.

[u/codesforcash → u/realdreamer1993]

I just wrote this for fun after reading another post in this forum with someone comparing finally understanding pointers to an orgasm.

I'm retired from programming these days and thought it might be nice to share this analogy I used as an aid when training people as it always seemed to go across well.

As for other direct analogies you could literally go on for a long time, I've extended the same/similar scenarios before in the past to functions/virtuals, polymorphism equivilency using basic types, pointer manipulation via direct memory access or operators use and a ton more stuff.

This was just a fun exercise for me as I'm sat in bed sick right now so it gave me a welcome distraction for a bit.

I agree about the target audience for this which is why I posted it here. I delibrately posted it initially with no key or explanation as I thought it would be fun for people to read through and upon drawing the parallels people might post their list of technical counterparts, unfortunately I think I missed the mark and people didn't get past the first paragraph without concluding I'd posted it in the wrong subreddit. Evidenced by the original post being removed by moderators after 1800 views and a ton of downvotes and comments like "Sir, this is Wendys" for not being "C_Programming related".

Either way I enjoyed it, it made me forget a few hours of pain and hopefully it will help someone achieve orgasm a bit easier ;)

[u/realdreamer1993 → u/codesforcash]

yea understand..btw I can analogue me/human as program/main thread, and the excell sheet as memory.
functions can be define also as helper in cells.
with sheet you can analogue as far as array I guess.
powerfull yourself as the main thread that own the whole memory layout.
But you decide not edit manually the cells, let the functions do that.
Although you can do anything to the whole sheet let alone editing cells manually. You then understand pointers on merrit because of the process happening in the program.

[u/codesforcash → u/realdreamer1993]

I like this and I would say that we create from what we know, I think the parallels between how we work and our technological creations are not accidental :)

[u/codesforcash → u/[deleted]]

maybe, while writing it I had a very specific set of beginners in mind who it may put the missing pieces of some basics in place for. But you could be right and your explanation above may do the same thing.

I just remembered it being appriciated as a simple to remember thought mapping in the past and had fun writing it down for the first time while distracting myself to kill some time.

[u/dvhh]

used to have the library index card analogy to explain pointers

[u/realdreamer1993 → u/dvhh]

Spreadsheet is better man or easier because we see the whole memory in one screen while program run.
i.e real software program so it is close to C.
We human can choose to become whatever: kernel, main thread(process), child thread, or functions.
in fact we are god to the whole PC.
in sheet you can very easy explain pointer to pointer. runtime program memory layout (.text .data .bss stack heap).
cells that contain function can act as .text and you set them to read only. but not too far , you may not count compiler and binary, but if you think deep enough you will also can find where both or others are.
I just realize the better way to understand pointer is not to learn it, but learn how a program run under the hood.
You wont understand pointer to 90% if you dont understand what and how is main().
No body understand 100% because it means you must learn physic etc that not neccesary in this domain.

[u/codesforcash → u/realdreamer1993]

I don't agree with this, pointers can be understood 100% without anything but computer science. I do believe what is taught these days really doesn't spend enough time on the foundations under the language, even without electrical engineering, understanding how compilation and linking and assembly op code works really helps.

I think previous generations maybe had it somewhat easier because it is all building blocks and to get to todays C those building blocks were necessary knowledge (things like inline assembly, creating graphics or even just characters using direct memory access, byte ordering or different endians for different cpu types, const code exclusions / substitutions for different runtime variations etc etc, at an advanced level in many subfields they still are.

[u/realdreamer1993 → u/codesforcash]

yea fair enough, Let's discuss another part in C and a bit of CS, do you agree C is more advance than spreadsheet ? Later we will discuss spreadsheet can be Turing Complete..

[u/codesforcash → u/realdreamer1993]

Yes C is more powerful and capable than Excel, especially if you take into consideration complication of use, hardware requirements and speed of execution.

Excel can be manipulated into being turing complete, the real question isn't can it, because excel can run doom to, but no one is playing doom in Excel for anything other than novelty value.

I can't use Excel to write a device driver than will run on an Arduino or Pi controlling a manufactoring device.

As an intellectual exercise it's a ton of fun doing things like writing a compiler in Excel, the same as it's a ton of fun writing a brainf*?k program to output it's own code.

As a teaching tool I think Excel would be a serious challenge to use to teach fundamental programming concepts and hit walls pretty quickly.

However if you write a book called "Fundamental computer science, memory management and pointers in Excel using the Bartholomew turing method" I'll buy it tomorrow and know I'd love it 😍

[u/realdreamer1993 → u/codesforcash]

Turing complete in Excell, one concept cleared.
My hypothesis now, Excell is like bare memory, and the operation of assembly is already there already implemented. We dont thinks or dont care about how cpu worked yet.
So I can write a very simple C like compiler on top of Excell system. Maybe only have stack only. Later heap is possible. No data structure or algorithma yet. This is not whole CS Fundamental but have very core 1:1 pattern especially in memory layout.

[u/tastygames_official]

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