Can enums be considered unsafe?

Question

Since I know each element of an enum has its own integer value, I tried this:

enum Foo {
    Red = 0,
    Blue = 1
};

int main(void) {
    enum Foo bar = 2;
    return 0;
}

And it... works. I looked further and I think it has the same minimum and maximum limits as int (I started experiencing UB after INT_MAX). At that point, isn't an enum no better than a set of #defines? I could also very well be wrong here, though.

Almost anything is unsafe if you use it in a completely wrong way — klutt, Jan 26 '21 at 14:54
In general, "safe" is a very relative term when it comes to C. — klutt, Jan 26 '21 at 14:56
Yeah, I guess that's true. I generally use `enum class`es from C++ but since I was making a C-only program I tried out `enum`s and discovered this. — mediocrevegetable1, Jan 26 '21 at 14:56
@mkrieger1 and @underscore_d your links are very helpful and I can see how `enum`s can still be important despite the issue, so thank you. — mediocrevegetable1, Jan 26 '21 at 14:59
I't just as unsafe as this `char s[10]; s[13]='x';` Does that makes arrays unsafe? — klutt, Jan 26 '21 at 15:00
@klutt yeah, that is true. I guess it all ultimately depends on the programmer not being stupid, in a way. — mediocrevegetable1, Jan 26 '21 at 15:01
In C yes. Many other languages goes a long way to protect you, but C does not. — klutt, Jan 26 '21 at 15:02
Some compilers have an option to automatically insert code to check the value. — the busybee, Jan 26 '21 at 15:25
I see. As @chqrlie mentioned in his answer, `clang`'s `-Weverything` alerts you when you assign a value to the `enum` that is not in the set of elements. — mediocrevegetable1, Jan 26 '21 at 15:30
This is pretty much an exact duplicate: [How to create type safe enums?](https://stackoverflow.com/questions/43043246/how-to-create-type-safe-enums). — Lundin, Jan 26 '21 at 15:30
@Lundin: merely a subset: your question addresses a more general issue, ie: one that works for values not known at compile time. — chqrlie, Jan 26 '21 at 15:32

chqrlie · Accepted Answer · 2021-01-26T15:20:12.207

3

An enum variable is an integer with a type large enough for the largest value defined in the enum definition, not necessarily int and potentially larger than int on some compilers. You can store int it any other value compatible with the storage type, no check is performed at run time.

If you increase your compiler's warning level, (eg: gcc -Wall -Wextra or clang -Weverything) you might get a warning about the value being different from all defined enumeration constants for the target enum.

For example clang produces these warnings when invoked with -Weverything:

test.c:7:20: warning: integer constant not in range of enumerated type 'enum Foo' [-Wassign-enum]
    enum Foo bar = 2;
                   ^
test.c:7:14: warning: unused variable 'bar' [-Wunused-variable]
    enum Foo bar = 2;

edited Jan 26 '21 at 15:20

answered Jan 26 '21 at 14:56

chqrlie

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I tried `-Wall -Wextra` in `gcc` and `clang`. Unfortunately, it does not produce a warning. – mediocrevegetable1 Jan 26 '21 at 15:09
@mediocrevegetable1: indeed `-Wall -Wextra` does not suffice, but `clang -Weverything` complains. – chqrlie Jan 26 '21 at 15:20
Ah yes, I'm getting the warning now. Thanks. – mediocrevegetable1 Jan 26 '21 at 15:23

Can enums be considered unsafe?

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