can't use "error" per-proccessor inside "if" pre-proccessor

Question

I have an enum:

typedef enum {
    ALI = 0,
    
    JOHN,
    CATI,

} NAMES_e;

As I know, as long as the enum has less than 256 items, it uses 1 byte of RAM. The storage is so important for me, so as I'm developing the program, this might become even more than 1000 names (it might!) and I should be aware of it (To control the storage).

1. So, I tried this in the main function:

if (sizeof(NAMES_e) == 1)
    #error "NAMES_eis more than 256"

2. It didn't work. I also used the same with #if:

#if (sizeof(NAMES_e) == 1)
    #error "NAMES_eis more than 256"
#endif

I got this error:

error: token is not a valid binary operator in a preprocessor subexpression

I checked this (My compiler doesn't support static_assert and this. Nothing special.

3. I did this:

int size = sizeof(NAMES_e);
size = 1; //Just to test.
#if (size == 1) 
    #error "NAMES_e is more than 256"
#endif

And the compiler doesn't compile the last one either. No errors while compiling.

What should I do?

You're mixing C statements and preprocessor statements. You can't do this. But anyway the size of an enum ist most likely the size of `int` even if the enum has less than 256 items. — Jabberwocky, May 26 '21 at 08:04
Please check the answers on the linked question again. You can implement your own static assertion if your compiler doesn't provide one. — user694733, May 26 '21 at 08:14
@Jabberwocky between 1-256, enums use 8 bits (one byte). more than 256, and less than 65536, they have to use 16 bits and so on. — Mohammad Kholghi, May 26 '21 at 08:21
@MohammadKholghi actually it depends on the platform. What is your platform? — Jabberwocky, May 26 '21 at 08:22
[This answer describes BUILD_BUG_ON macro.](https://stackoverflow.com/a/18511691/694733) [This CT_ASSERT macro.](https://stackoverflow.com/a/19355668/694733) And there are many more similar answers in that question. — user694733, May 26 '21 at 08:32
@user694733 I checked this site https://scaryreasoner.wordpress.com/2009/02/28/checking-sizeof-at-compile-time/ (the question redirected me), and it works for `%`, but I want it to work with `!=`. — Mohammad Kholghi, May 26 '21 at 09:06
@user694733 Problem solved. Its answer is a little bit different. Thanks a lot. — Mohammad Kholghi, May 26 '21 at 09:27

score 0 · Answer 1 · answered May 26 '21 at 09:34

We can use a small trick! First, I should mention that the sizeof(char[0]) doesn't compile and throws an error. So, we take advantage of it!:

#define CHECK_SIZE(condition) ((void)sizeof(char[2 - (condition)]))
//...
CHECK_SIZE(sizeof(NAMES_e));

How does it work? If the NAMES_e is 2 bytes, it tries to check the sizeof(char[0]), which we talked about.

For more information, read this and this.

can't use "error" per-proccessor inside "if" pre-proccessor

1 Answers1