C/C++ Create an enum with negative values, without

Question

For example in C/C++, I would have the code:

typedef enum fruits{
   apple,
   banana,
   lemon,
   orange
} fruit_t;

Which would be equivalent to:

typedef enum fruits{
   apple = 0,
   banana = 1,
   lemon = 2,
   orange = 3
} fruit_t;

However, I would like the values to be negative, so they do not conflict with anything else. I could do it like this:

typedef enum fruits{
   apple = -1,
   banana = -2,
   lemon = -3,
   orange = -4
} fruit_t;

But if I would like to add another fruit, I have to assign another value, and if I put one inbetween, I have to renumber most of it. Is there an easier way of doing this?

Answer 1

First of all, there is no such thing as "C/C++", those are two different languages. You can't look at C as a subset of C++; some legal C code won't compile as a C++ program.

If you can use C++11, I suggest you use enum class, which gives you strongly typed enumerations:

enum class Traffic {red , yellow, green};
Traffic t = Traffic::red;

if ( t == Traffic::red )  // to test the value of an enum 

This way you can guarantee no clashing, because the compiler won't let you do any comparison with an integer or with a variable of a different enum type.

You can only compare against a variable of the same enum type, or use the binary scope resolution operator as in the example.

Another advantage of this enum class is that you can set the size of your enum. You can use any signed or unsigned integer type like this:

enum class Traffic : char { red, yellow, green }

When a type is not specified, the default int is assumed.

Note that you need a compiler with C++11 support for this.

Answer 2

It is also quite common to declare the errors as usual enumerations and use the negative value of it, like:

typedef enum fruits_e {
    APPLE=1,
    BANANA,
    LEMON,
    ORANGE
} fruit_t;

fruit_t eat_fastfood(void) {
    if (! me->healthy())
        return -APPLE;
}

Answer 3

In C you only need to number the top one and subsequent entries will be the previous entry +1, this is covered in the C99 draft standard section 6.7.2.2 Enumeration specifiers which says (emphasis mine):

[...]An enumerator with = defines its enumeration constant as the value of the constant expression. If the first enumerator has no =, the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant.[...]

and the wording is similar in the C++ draft standard section 7.2 Enumeration declarations paragraph 2.

So just do something similar to the following:

#define MIN -4

typedef enum fruits{
   orange = MIN,
   lemon,  
   banana,
   apple,
} fruit_t;

Answer 4

I think you can order it anyway you like.

If you are not giving any value then next enum will take +1 of its previous value. If even fist enum is not having any value then it will start from 0.

But good practice is to have it in right order.

Answer 5

Start with INT_MIN on top like this:

#include <limits.h>

enum fruits
{
  orange = INT_MIN,
  lemon, 
  ...
}

Per "5.2.4.2.1 Sizes of integer types " and/or "Annex E/1" of the C11 Standard INT_MIN is at least -32767.

INT_MIN gets defined by including <limits.h>.

Answer 6

Add news ones at the top. You still have to number the topmost one.

enum fruit
{
   orange = -4,
   lemon,
   banana,
   apple,
};