I am looking through some C source code and I don't understand the following part
#if 1 typedef unsigned short PronId; typedef unsigned short LMId; # define LM_NGRAM_INT #else typedef unsigned int LMId; typedef unsigned int PronId; # undef LM_NGRAM_INT #endif Why would someone do #if 1? Isn't it true that only the first block will ever be processed?
Yes.. Only the first block will be processed --- until someone changes the 1 to a 0. Then the other block will be compiled. This is a convenient way to temporary switch blocks of code in and out while testing different algorithms.
So that one can quickly choose which part to compile by changing the #if 1 to #if 0.
One of the fundamental properties of software is that computer program is cheap to modify.
That's why certain code is written in such a way that it will make modification easier. That's why they need various patterns, like "interface", or "proxy".
And that's why you sometimes see weird constructs like #if 1-#else-#endif, an only purpose of which is to easily switch the part of code that will be compiled, by small effort: changing 1 to 0.
Computer program is cheap to modifyI put that in my code when I need to test different set of parameters. Usually my product will ship with different defaults than what I can work with in a debug environment, so I put the shipping defaults in a #if 1 and the debug defaults in the #else with a #warning to warn me it's being built with debug defaults.
For experimenting with various code paths.
I'm actually using it as a kludge to make code folding easier; if I wrap a section of code in an #if 1 ... #endif, I can fold it in my editor. (The code in question is very macro-heavy, and not written by me, so more traditional ways of making a huge block of code manageable won't work.)
The cleaner way of doing it is probably doing something like:
#if ALGO1 #else #endif But, you will have to pass in ALGO1 to the compiler args somewhere...for example in a makefile, you need to add -DALGO1=1 (if no 1 is provided, 1 is assumed). Ref: http://www.amath.unc.edu/sysadmin/DOC4.0/c-compiler/user_guide/cc_options.doc.html
This is more work...so, usually, for quick checks, #if 1 is used. And in some cases, forgotten and left behind as well :-)
It's another way of saying for #if true it was most likely a result of code that was previously checking for another symbol then refactored to always be true.
