How can I use "else if" with the preprocessor #ifdef?

Question

In my project, the program can do one thing of two, but never both, so I decided that the best I can do for one class is to define it depending of a #define preprocessor variable. The following code can show you my idea, but you can guess that it does not work:

#ifdef CALC_MODE
typedef MyCalcClass ChosenClass;
#elifdef USER_MODE
typedef MyUserClass ChosenClass;
#else
static_assert(false, "Define CALC_MODE or USER_MODE");
#endif

So I can do

#define CALC_MODE

right before this.

I can resign the use of static_assert if needed. How can I do this?

You can use #if defined(CALC_MODE) and #elif defined(USER_MODE) — Retired Ninja
– Retired Ninja, Commented Aug 7, 2021 at 22:47
Are you looking for $elif defined(...)? You may also find the #error directive interesting. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Aug 7, 2021 at 22:48
@StoryTeller-UnslanderMonica Right, actually better than static_assert in this case, because it will always be false — Jonnas Kraft
– Jonnas Kraft, Commented Aug 7, 2021 at 22:59

Adrian Mole · Accepted Answer · 2021-08-07 23:10:28Z

45

Here's a suggestion, based largely on comments posted to your question:

#if defined(CALC_MODE)
    typedef MyCalcClass ChosenClass;
#elif defined(USER_MODE)
    typedef MyUserClass ChosenClass;
#else
    #error "Define CALC_MODE or USER_MODE"
#endif

answered Aug 7, 2021 at 23:10

community wiki

Adrian Mole

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5 Comments

Eric Postpischil Over a year ago

defined is an operator, not a function, like sizeof, so the parentheses are unnecessary and are just clutter. It is better that students see it is an operator that directs the compiler to do something rather than that it is a function that is called.

Adrian Mole Over a year ago

@Eric It's a Community Wiki, so feel free to edit. However, I think you'll find a near 50:50 split on the use of sizeof Eric versus sizeof(Eric).

Silvio Mayolo Over a year ago

Yeah, definitely not cut-and-dry. I personally always use parens on sizeof and defined, just because it looks a lot cleaner in my mind. There are lots of folks in both camps.

Adrian Mole Over a year ago

I've done most of my programming on the Windows platform using Microsoft tools and code, so maybe I've picked up their habits.

Алексей Неудачин Over a year ago

you forget about say sizeof(int) . you can't do this no-braces way haha

Joachim Mairböck · Accepted Answer · 2023-07-10 12:49:52Z

13

#elifdef and #elifndef are officially supported since C23 and C++23.

But these standards are not yet released (as of July 2023) and the new features are not yet widely adopted by compilers, so you are still better off with #if defined() for now.

answered Jul 10, 2023 at 12:49

Joachim Mairböck

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