Question

Is there a compile-time way to detect / prevent duplicate values within a C/C++ enumeration?

The catch is that there are multiple items which are initialized to explicit values.

Background:

I’ve inherited some C code such as the following:

#define BASE1_VAL    (5)
#define BASE2_VAL    (7)

typedef enum
{
  MsgFoo1A = BASE1_VAL,       // 5
  MsgFoo1B,                   // 6
  MsgFoo1C,                   // 7
  MsgFoo1D,                   // 8
  MsgFoo1E,                   // 9
  MsgFoo2A = BASE2_VAL,       // Uh oh!  7 again...
  MsgFoo2B                    // Uh oh!  8 again...
} FOO;

The problem is that as the code grows & as developers add more messages to the MsgFoo1x group, eventually it overruns BASE2_VAL.

This code will eventually be migrated to C++, so if there is a C++-only solution (template magic?), that’s OK — but a solution that works with C and C++ is better.

Answer 1

There are a couple ways to check this compile time, but they might not always work for you. Start by inserting a “marker” enum value right before MsgFoo2A.

typedef enum
{
    MsgFoo1A = BASE1_VAL,
    MsgFoo1B,
    MsgFoo1C,
    MsgFoo1D,
    MsgFoo1E,
    MARKER_1_DONT_USE, /* Don't use this value, but leave it here.  */
    MsgFoo2A = BASE2_VAL,
    MsgFoo2B
} FOO;

Now we need a way to ensure that MARKER_1_DONT_USE < BASE2_VAL at compile-time. There are two common techiques.

Negative size arrays

It is an error to declare an array with negative size. This looks a little ugly, but it works.

extern int IGNORE_ENUM_CHECK[MARKER_1_DONT_USE > BASE2_VAL ? -1 : 1];

Almost every compiler ever written will generate an error if MARKER_1_DONT_USE is greater than BASE_2_VAL. GCC spits out:

test.c:16: error: size of array ‘IGNORE_ENUM_CHECK’ is negative

Static assertions

If your compiler supports C11, you can use _Static_assert. Support for C11 is not ubiquitous, but your compiler may support _Static_assert anyway, especially since the corresponding feature in C++ is widely supported.

_Static_assert(MARKER_1_DONT_USE < BASE2_VAL, "Enum values overlap.");

GCC spits out the following message:

test.c:16:1: error: static assertion failed: "Enum values overlap."
 _Static_assert(MARKER_1_DONT_USE < BASE2_VAL, "Enum values overlap.");
 ^