Question

we use VS 2008

there is a big enum, which is populated by many developers

this enum has a type __int64 (a Microsoft extension), and I want to make compiler complain about non unique values in enum.

if it was a usual enum I would do like this:

enum E1
{
    E11 = 0x01F00,
    E12 = 0x01F00,
    E13
};
#pragma warning(push)
#pragma warning(error: 4061)
#pragma warning(error: 4062)
void F(E1 e1)
{
    switch (e1)
    {
    case E11:
    case E12:
    case E13:
        return;
    }
}
#pragma warning(pop)

and the function F would have an error, if E1 has 2 same values

and it would have another error, if a developer forgot to add a new value to switch

but my enum has type __int64 (or long long)

and when I try do the same switch for E1 e1 it truncates values and complains on values, wich difference is either 0x100000000, or 0x200000000 ….

if I cast e1 to __int64, the compiler does not complain, if a developer forgets to add a new value to the switch (so the whole check function becomes useless)

the question: does someone know what I can do about it? or maybe VS 2008 (or C++) has another instrument to ensure enum : __int64 has only unique values?

Answer 1

From your comment I’ll assume that you don’t have aggregate (combined) flags within the enum itself. In that case you can use two enums to make it much harder to make mistakes. You can still subvert the compiler but I assume that’s not the real problem here.

enum Bit_Index
{
    FLAG1_INDEX,
    FLAG2_INDEX,
    FLAG_FANCY_INDEX,
    LAST_INDEX
};

#define DECLARE_BIT_VALUE(att) att##_VALUE = 1ULL << att##_INDEX
enum Bit_Value
{
    DECLARE_BIT_VALUE(FLAG1),
    DECLARE_BIT_VALUE(FLAG2),
    DECLARE_BIT_VALUE(FLAG_FANCY),

    // Declared NOT using the macro so we can static assert that we didn't forget
    // to add new values to this enum.
    LAST_BIT   // Mainly a placeholder to prevent constantly having to add new commas with new ids.
};
#undef DECLARE_BIT_VALUE

Then in an implementation file you static_assert to make sure the enums don’t get misaligned:

// Make sure to the best of our abilities that we didn't mismatch the index/bit enums.
BOOST_STATIC_ASSERT((LAST_BIT - 1) == (1U << (LAST_INDEX - 1)));