3 ms·

Metaprogramming custom control structures in C

60 points by AndrewDucker 15y ago

nicolast 15y ago
I did something quite similar to this in [1] and [2] some time ago. [1] https://github.com/Incubaid/crakoon/blob/master/src/arakoon.h#L277 https://github.com/Incubaid/crakoon/blob/master/src/arakoon.... [2] https://github.com/Incubaid/crakoon/blob/master/src/arakoon.h#L316 https://github.com/Incubaid/crakoon/blob/master/src/arakoon....
ay 15y ago
As someone who had to single-step through this kind of code in gdb, and who had to use the series of fgreps because the cscope is not smart enough to digest these constructs, I can only suggest - weigh the pros and cons before using this technique, especially the fancier parts of it.
- wcarss 15y ago
  (In agreement) It is tough to imagine the mental investment of another if-statement or more setup/teardown exceeding what's required to notice, understand, and debug one of these structures. You get slightly prettier code at a tremendous possible cost. It feels like making people aware that they can use bazookas to kill butterflies. People are hopefully reasonable, but it can go very wrong. All that said, cool.
- mirsadm 15y ago
  Agreed. I would be very very careful about using this. The headaches it could cause far outweigh the (arguable) benefits.
- fancydriving 15y ago
  I think a good rule of thumb is to only use the preprocessor to generate code that you would be comfortable writing yourself. Spaghetti produced by macros is still spaghetti. The article reminded me of queue.h from BSD (http://www.openbsd.org/cgi-bin/cvsweb/src/sys/sys/queue.h?rev=1.35;content-type=text%2Fplain http://www.openbsd.org/cgi-bin/cvsweb/src/sys/sys/queue.h?re...), but those macros expand to readable code.
  - ay 15y ago
    These macros are nothing compared to what I consider a real abuse of the preprocessor - a whole functional language implemented just by that! Search for "chaos-pp" in this entry (which has plenty of other wonders too!): http://stackoverflow.com/questions/652788/what-is-the-worst-real-world-macros-pre-processor-abuse-youve-ever-come-across http://stackoverflow.com/questions/652788/what-is-the-worst-...
jff 15y ago
Oh boy--macros! Just say no to crack, kids, or you'll end up like the Linux kernel.
raingrove 15y ago
#define unless(x) if(!(x))
halayli 15y ago
Better called, evil control structures. These macros add little value and makes it harder to understand the code.

60 points by AndrewDucker 15y ago

I did something quite similar to this in [1] and [2] some time ago. [1] https://github.com/Incubaid/crakoon/blob/master/src/arakoon.h#L277 https://github.com/Incubaid/crakoon/blob/master/src/arakoon.... [2] https://github.com/Incubaid/crakoon/blob/master/src/arakoon.h#L316 https://github.com/Incubaid/crakoon/blob/master/src/arakoon....

ay 15y ago

As someone who had to single-step through this kind of code in gdb, and who had to use the series of fgreps because the cscope is not smart enough to digest these constructs, I can only suggest - weigh the pros and cons before using this technique, especially the fancier parts of it.

wcarss 15y ago
(In agreement) It is tough to imagine the mental investment of another if-statement or more setup/teardown exceeding what's required to notice, understand, and debug one of these structures. You get slightly prettier code at a tremendous possible cost. It feels like making people aware that they can use bazookas to kill butterflies. People are hopefully reasonable, but it can go very wrong. All that said, cool.
mirsadm 15y ago
Agreed. I would be very very careful about using this. The headaches it could cause far outweigh the (arguable) benefits.
fancydriving 15y ago
I think a good rule of thumb is to only use the preprocessor to generate code that you would be comfortable writing yourself. Spaghetti produced by macros is still spaghetti. The article reminded me of queue.h from BSD (http://www.openbsd.org/cgi-bin/cvsweb/src/sys/sys/queue.h?rev=1.35;content-type=text%2Fplain http://www.openbsd.org/cgi-bin/cvsweb/src/sys/sys/queue.h?re...), but those macros expand to readable code.
- ay 15y ago
  These macros are nothing compared to what I consider a real abuse of the preprocessor - a whole functional language implemented just by that! Search for "chaos-pp" in this entry (which has plenty of other wonders too!): http://stackoverflow.com/questions/652788/what-is-the-worst-real-world-macros-pre-processor-abuse-youve-ever-come-across http://stackoverflow.com/questions/652788/what-is-the-worst-...

jff 15y ago

Oh boy--macros! Just say no to crack, kids, or you'll end up like the Linux kernel.

raingrove 15y ago

#define unless(x) if(!(x))

halayli 15y ago

Better called, evil control structures. These macros add little value and makes it harder to understand the code.