Hi
IIUC, the EnumUnion has no tag, which means you could store an Enum1 and retrieve an Enum2 without any warning. Is that correct?
In UnionModel, there's:
static const size_t size = head::size > _tail::size ? head::size : _tail::size;
Almost, the EnumUnion has a tag, but this is only used to find the
specific union. Consider:
CompressedEnums<
unsigned,
EnumUnion
double_union;
Here I have two union, where three enums shares the same storage.
which makes sense because a union just has to be large enough to store the largest enumeration. However, in the case of StructModel_r, I don't understand:
static const size_t size = head::size * _tail::size;
around line 200. I would think that there would just be additions since you want to store one value after another. That's why, in my previous post, in template sum_bits, there's:
, integral_c
Could you explain why multiplication instead of addition is used to calculate the size in StructModel_r?
This is because the offset is not really an offset, I just could not come up with a better name. Here is the explanation: Assume we need to store 5 different enumerated types which each represents 3 different values ( like the A, B C). These 5 enums can all together represent 3^5 = 243 different values ( AAAAA, AAAAB, AAAAC, AAABA, AAABB ...). As 243 is less than 256 ( all the different values in a char), it is possible to store the values in a char. But in a binary number system 2 bits are required to store one tri-state value. If we use 3 * 5 bits we end up with 15 bits ( more than one char) which is not the most efficient encoding. Therefore we encode this in a base_3 number system which is defined by: ... d_2 * n^2 + d_1 * n^1 + d_0 * n^0, where d is the digits we want to encode, and n is 3 because all the enums are tri-states. Example: Encode BCACB -> 12021 1*3^4 + 2*3^3 + 0*3^2 + 2*3^1 + 1*3^0 = 142 To extract enum number 4 the reverse operation must be done: (142 / 3^3) % 3 = 2 If different enums are with different sizes are used then n in the equation above will be different for each digit. Then to set digit number 4 one has to multiply n_0 * n_1 * n_2 * n_3 and then use this as base. Therefore the multiplications.
Could you explain why multiplication instead of addition is used to calculate the size in StructModel_r? Currently no, but I would like to add this. Also a range check on the array access.
Best regards Allan W. Nielsen