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

While we do not have full on reflection, you can solve this problem if you can relist the enumeration values.

Somewhere this is declared:

enum E { A = 0, B = 0 };

elsewhere, we build this machinery:

template
struct first_not_same_as_rest : std::true_type {};
template
struct first_not_same_as_rest : std::integral_constant< bool,
  (s0 != s1) && first_not_same_as_rest< S, s0, s... >::value
> {};


template
struct is_distinct : std::true_type {};

template
struct is_distinct : std::integral_constant< bool,
  std::is_distinct::value &&
  first_not_same_as_rest< S, s0, s... >::value
> {};

Once you have that machinery (which requires C++11), we can do the following:

static_assert( is_distinct< E, A, B >::value, "duplicate values in E detected" );

and at compile time we will ensure that no two elements are equal.

This requires O(n) recursion depth and O(n^2) work by the compiler at compile time, so for extremely large enums this could cause problems. A O(lg(n)) depth and O(n lg(n)) work with a much larger constant factor can be done by sorting the list of elements first, but that is much, much more work.

With the enum reflection code proposed for C++1y-C++17, this will be doable without relisting the elements.