Detect when multiple enum items map to same value

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 initialize

Answer 1

I didn't completely like any of the answers already posted here, but they gave me some ideas. The crucial technique is to rely on Ben Voight's answer of using a switch statement. If multiple cases in a switch share the same number, you'll get a compile error.

Most usefully to both myself and probably the original poster, this doesn't require any C++ features.

To clean things up, I used aaronps's answer at How can I avoid repeating myself when creating a C++ enum and a dependent data structure?

First, define this in some header someplace:

#define DEFINE_ENUM_VALUE(name, value)      name=value,
#define CHECK_ENUM_VALUE(name, value)       case name:
#define DEFINE_ENUM(enum_name, enum_values) \
    typedef enum { enum_values(DEFINE_ENUM_VALUE) } enum_name;
#define CHECK_ENUM(enum_name, enum_values) \
    void enum_name ## _test (void) { switch(0) { enum_values(CHECK_ENUM_VALUE); } }

Now, whenever you need to have an enumeration:

#define COLOR_VALUES(GEN) \
    GEN(Red, 1) \
    GEN(Green, 2) \
    GEN(Blue, 2)

Finally, these lines are required to actually make the enumeration:

DEFINE_ENUM(Color, COLOR_VALUES)
CHECK_ENUM(Color, COLOR_VALUES)

DEFINE_ENUM makes the enum data type itself. CHECK_ENUM makes a test function that switches on all the enum values. The compiler will crash when compiling CHECK_ENUM if you have duplicates.