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#if vs #if defined vs #ifdef directives - how are they different from each other? (lemmy.ml)
submitted 4 days ago by velox_vulnus@lemmy.ml to c/c_programming@lemmy.ml
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[-] filtoid@lemmy.ml 2 points 4 days ago

I'm going to take a shot at answering this, but please bear in mind that it's been a long time since I looked into the C pre-parser.

#if allows for arbitrary Boolean logic, eg #if build_env "local"

#if defined us to see if a thing has been defined already, quite often used to make sure that a header file is only imported once, redeclaration of headers is a compiler error. Eg. #if !defined(__SOME_UNIQUE_FILE_IDENTIFIER) (then define the class) then write your #endif

#ifdef wasn't always standard and was added later in the ANSI spec be a keyword, it's shorthand for the same thing as #if defined. -

This is my understanding anyway, I'm going with the principal that someone who knows more will be more likely to post a rebuttal (which I encourage).

[-] smpl@discuss.tchncs.de 1 points 4 days ago

The standard agrees with you that #ifdef and #ifndef are equal to #if defined and #if !defined, but your #if build_env "local" example does not make much sense the me. #if only takes constant expressions which can be evaluated at translation time. It can't compare strings.

[-] filtoid@lemmy.ml 4 points 4 days ago* (last edited 4 days ago)

You are correct, I had misremembered how it works. It can evaluate build arguments, but they have to be numerical. However you can define new variables (in the pre-processor, not in code - ie, after a #) to replace those numbers, to make the intent clearer. Eg.

#define DEBUG_1 1

#define DEBUG_2 2

#if DEBUG_LEVEL >= DEBUG_1

Then pass the build arg DEBUG_LEVEL at compile time

More info here:

https://learn.microsoft.com/en-us/cpp/preprocessor/hash-if-hash-elif-hash-else-and-hash-endif-directives-c-cpp?view=msvc-170

Edit: formatting of code snippet

this post was submitted on 29 Sep 2024
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