3 ms·

I expect many people know this one, but it's a useful teaching aid when understanding the relationship between arrays and pointers int array[10]; *(array+1

I expect many people know this one, but it's a useful teaching aid when understanding the relationship between arrays and pointers int array[10]; *(array+1) = 56; array[2] = 4; 3[array] = 27; The first two are obvious, but the third is also legal. It works because array indexing is just sugar for pointer arithmetic, so array[2]=4 is identical in meaning to *(array+2)=4. Therefore 3[array]=27 is identical to *(3+array)=27 and so is legal. But just because you can doesn't mean you should.

macintux 2y ago
The best, most entertaining book I've ever read on C covered that (unless I'm misremembering, but I doubt it): Expert C Programming. https://www.goodreads.com/book/show/198207.Expert_C_Programming https://www.goodreads.com/book/show/198207.Expert_C_Programm...
- dualogy 2y ago
  I'm already liking that one! Page 5 quote: > There is one other convention — sometimes we repeat a key point to emphasize it. In addition, we sometimes repeat a key point to emphasize it. One more quote and I'll stop: > ctime() converts its argument into local time, which will vary from GMT, depending on where you are. California, where this book was written, is eight hours behind London, and several years ahead
WalterBright 2y ago
> The first two are obvious, but the third is also legal. D doesn't have that bug! In 44 years of C programming, I've never encountered a legitimate use for the 3rd. (Other than Obfuscated C, that is.))
- mhandley 2y ago
  Agreed - I've only been programming C for 38 years but I've also never found a legitimate use. However I have used it to illustrate a point when teaching C to beginners - it looks so odd they tend to remember it.
- WolfeReader 2y ago
  It's not a bug. You're seeing the difference between "this is how you're taught to access arrays" and "this is how array access actually works".
  - WalterBright 2y ago
    Since the Standard specifies what that does, pedantically it is not a bug. Ok. But I call it a bug because it has no use and just pointlessly confuses people.
  - im3w1l 2y ago
    Well it could (and I agree with WalterBright that it should) have been disallowed. a[b] being implemented as an early stage rewrite rule expanding to *(a+b) is an uninteresting implementation detail. And I doubt it is even implemented that way in modern compilers anyway. It certainly can't be in C++ as a[b] and b[a] mean different things when [] is overloaded.
    - WolfeReader 2y ago
      That "uninteresting implementation detail" is actually of grave importance when it comes to understanding how buffer overflow attacks work. I hate to think anyone would put C code into production without understanding this.
  - kragen 2y ago
    You seem to be lecturing the author of one of the most prominent early C compilers on how array access actually works in C.
    - WolfeReader 2y ago
      Yep.

macintux 2y ago

The best, most entertaining book I've ever read on C covered that (unless I'm misremembering, but I doubt it): Expert C Programming. https://www.goodreads.com/book/show/198207.Expert_C_Programming https://www.goodreads.com/book/show/198207.Expert_C_Programm...

dualogy 2y ago
I'm already liking that one! Page 5 quote: > There is one other convention — sometimes we repeat a key point to emphasize it. In addition, we sometimes repeat a key point to emphasize it. One more quote and I'll stop: > ctime() converts its argument into local time, which will vary from GMT, depending on where you are. California, where this book was written, is eight hours behind London, and several years ahead

WalterBright 2y ago

> The first two are obvious, but the third is also legal. D doesn't have that bug! In 44 years of C programming, I've never encountered a legitimate use for the 3rd. (Other than Obfuscated C, that is.))

mhandley 2y ago
Agreed - I've only been programming C for 38 years but I've also never found a legitimate use. However I have used it to illustrate a point when teaching C to beginners - it looks so odd they tend to remember it.
WolfeReader 2y ago
It's not a bug. You're seeing the difference between "this is how you're taught to access arrays" and "this is how array access actually works".
- WalterBright 2y ago
  Since the Standard specifies what that does, pedantically it is not a bug. Ok. But I call it a bug because it has no use and just pointlessly confuses people.
- im3w1l 2y ago
  Well it could (and I agree with WalterBright that it should) have been disallowed. a[b] being implemented as an early stage rewrite rule expanding to *(a+b) is an uninteresting implementation detail. And I doubt it is even implemented that way in modern compilers anyway. It certainly can't be in C++ as a[b] and b[a] mean different things when [] is overloaded.
  - WolfeReader 2y ago
    That "uninteresting implementation detail" is actually of grave importance when it comes to understanding how buffer overflow attacks work. I hate to think anyone would put C code into production without understanding this.
- kragen 2y ago
  You seem to be lecturing the author of one of the most prominent early C compilers on how array access actually works in C.
  - WolfeReader 2y ago
    Yep.