Added indexable properties on enums

[Shelim]

• _to_index which always return value 0... size-1
  (even if enums are not sequential)
• _from_index used for other way around
• _from_index_nothrow no throw version
• _from_index_unchecked returns invalid enum
  in case arg>=size

Code and test cases added.
No documentation updates (my English is too poor for that)

Usage case:

BETTER_ENUM(Enum, int, A = 128, B = 256, C = 1024)
for(int i = 0; i < Enum::_size(); i++)
{
    Enum foo = Enum::_from_index_unchecked(i);
    printf("%d enum is %s \n", i, foo._to_string());
}

[Shelim]

Sorry, older version pushed for some reason o.O I'll fix that asap

[Shelim]

Uff... I believe it is done now. Hard weekend...

[Shelim]

Fail is in clang-3.3 and is not related to that code

[aantron]

At first glance, it looks like we should expose _from_value_loop, which is what the new _to_index does. For the rest of the functions, the same functionality can be used by the user with:

MyEnum::_values()[index]

and the user can implement their own error handling for the case where the index is out of range, since there is public MyEnum::_size().

Does that serve your needs?

If so, I would suggest cutting this PR down to include only _to_index, by making _from_value_loop pubic and renaming it. In the documentation for _to_index, we can tell users how to perform the reverse operation by using _values()[index].

[aantron]

I recommend not calling this ..._loop, because it doesn't loop over the indexes, but just does a direct lookup. In fact, it looks like just the implementation of _from_index_nothrow, so I suggest moving it there.

[aantron]

Looking more closely at the use case, I think it is completely covered by direct access to _values()[index].

[Shelim]

yup, but I would very much prefer symmetrical API - it is less confusing - if we are to expose method _to_index :)

Another, more pratctial usage case:

    BETTER_ENUM(my_enum, int, A = 65536, B = 65537, C = 536870912)
    std::bitset<my_enum._size()> bitset; // Compact bitset
    bitset.set(+(my_enum::foo)._to_index());

[aantron]

Okay, thanks for the PR!