Re: enums and switch vs function calls

On 05/04/2025 17:53, Thiago Adams wrote:

Em 4/5/2025 7:29 AM, Richard Harnden escreveu:

If I have:
>
enum colour {RED, GREEN, BLUE}
>
int colour_to_hex(enum colour colour)
{
     switch (colour)
     {
         RED: return 0xff0000;
         GREEN: return 0x00ff00;
         // BLUE: return 0x0000ff;
     }
>
     ...
}
>
... then the compiler will warn me that I've missed a case in the switch statement.  Which is good and very helpful.
>
But if I do:
>
int hex = colour_to_hex(10);
>
... then the compiler doesn't complain that 10 is not in the enum.
>
Why? Surely the compiler can tell.

 Sometimes enums are used as bit set.
 f(BIT1 | BIT2)
 this situation would generate a lot of warning because BIT1|BIT2 is not part of the enum, just BIT1 and BIT2 separately.
 One way that this could be fixed is adding an attribute
 [[bitset]]
enum E{ BIT1 = 2, BIT2 = 4};
 Then the compiler would not generate a warning in this case.
 

gcc has the "flag_enum" attribute precisely for this purpose. Standardising it in the C standards would be nice, but the C standard does not currently have much that is merely about improving compiler warnings.