How to get warning when comparing two enum value

In this small DEMO in Objective-C:

first enum:

typedef NS_ENUM(NSUInteger, Day) {
DaySunday,
DayMonday,
DayTuesday
};


second enum:

typedef NS_ENUM(NSUInteger, Month) {
MonthJanuary,
MonthFebruary,
MonthMarch,
MonthApril
};


when comparing :

Day sunday = DaySunday;
Month january = MonthJanuary;
if (sunday == january) {
NSLog(@"case1 with warning");
}
if (DaySunday == january) {
NSLog(@"case2 without warning");
}


and Xcode snapshot:

so how could i get a warning in case2?

3 Answers
3

Enumeration types in (Objective-)C are very weak types. By the C Standard every enumeration constant (your january etc.) has an integer type, not the type of the enumeration. Furthermore a value of enumeration type is implicitly converted to an integer type when needed.

january

Clang is giving you a warning when both operands are of enumeration type, and it is only a warning as by the C Standard the comparison is a correct when between integer values.

In your DaySunday == january the left operand has integer type, the right operand is implicitly convert to integer type, so again this is perfectly legal and correct Standard C. Clang could choose to issue a warning, why it does not is probably down to a design decision, or consequence of the design, on Clang internals.

DaySunday == january

Be thankful Clang often gives warnings where Standard C does not require them, however you cannot rely on it showing all the traps in C.

To address your issue you can cast the literal to the enum type if you wish, (Day)DaySunday == january, but you might reasonably decide this makes C look even worse ;-)

(Day)DaySunday == january

I'm not sure why this behavior is happening, but it's strange and cool. To get the warning, you have to cast DaySunday as type Day explicitly.

DaySunday

Day

if ((Day)DaySunday == january) {
NSLog(@"case2 without warning");
}


Explicitly casting january as Month won't trigger the warning, so it looks like the static analyzer is correctly treating january as a Month type (because you declared it that way), but is implicitly converting DaySunday to make the comparison work.

january

Month

january

Month

DaySunday

To be fair, the warning in the first case is actually not the ideal behavior, because both Day and Month are NSUIntegers and therefore are comparable. As you observe when you run this code, both comparisons are true, meaning the warning isn't actually meaningful.

Day

Month

You have to change enum to int for removing Warning

if ((int)sunday == (int)january) {
NSLog(@"case1 with warning");
}


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