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

Here's a solution using X macro without Boost. First define the X macro and its helper macros. I'm using this solution to portably make 2 overloads for the X macro so that you can define the enum with or without an explicit value. If you're using GCC or Clang then it can be made shorter

#define COUNT_X_ARGS_IMPL2(_1, _2, count, ...) count
#define COUNT_X_ARGS_IMPL(args) COUNT_X_ARGS_IMPL2 args
#define COUNT_X_ARGS(...) COUNT_X_ARGS_IMPL((__VA_ARGS__, 2, 1, 0))

/* Pick the right X macro to invoke. */
#define X_CHOOSE_HELPER2(count) X##count
#define X_CHOOSE_HELPER1(count) X_CHOOSE_HELPER2(count)
#define X_CHOOSE_HELPER(count)  X_CHOOSE_HELPER1(count)

/* The actual macro. */
#define X_GLUE(x, y) x y
#define X(...) X_GLUE(X_CHOOSE_HELPER(COUNT_X_ARGS(__VA_ARGS__)), (__VA_ARGS__))

Then define the macro and check it

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

// Enum values
#define MY_ENUM             \
X(MsgFoo1A, BASE1_VAL)      \
X(MsgFoo1B)                 \
X(MsgFoo1C)                 \
X(MsgFoo1D)                 \
X(MsgFoo1E)                 \
X(MsgFoo2A, BASE2_VAL)      \
X(MsgFoo2B)

// Define the enum
#define X1(enum_name)               enum_name,
#define X2(enum_name, enum_value)   enum_name = enum_value,
enum foo
{
    MY_ENUM
};
#undef X1
#undef X2

// Check duplicates
#define X1(enum_name)               case enum_name: break;
#define X2(enum_name, enum_value)   case enum_name: break;
static void check_enum_duplicate()
{
    switch(0)
    {
        MY_ENUM
    }
}
#undef X1
#undef X2

Use it

int main()
{
// Do something with the whole enum
#define X1(enum_name)               printf("%s = %d\n", #enum_name, enum_name);
#define X2(enum_name, enum_value)   printf("%s = %d\n", #enum_name, enum_value);
    // Print the whole enum
    MY_ENUM
#undef X1
#undef X2
}