Is it possible to use array of bit fields? - c++

Question

I am curious to know, Is it possible to use array of bit fields? Like:
struct st
{
unsigned int i[5]: 4;
};

Answer 1

No, you can't. Bit field can only be used with integral type variables.
C11-§6.7.2.1/5
A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.
Alternatively you can do this
struct st
{
unsigned int i: 4;
} arr_st[5];
but its size will be 5 times the size of a struct (as mentioned in comment by #Jonathan Leffler) having 5 members each with bit field 4. So, it doesn't make much sense here.
More closely you can do this
struct st
{
uint8_t i: 4; // Will take only a byte
} arr_st[5];

Answer 2

C does not support arrays of bit-fields, so the short answer is no.
For very large arrays, it might be worthwhile to pack values, 2 per byte, this way:
#define ARRAY_SIZE 1000000
unsigned char arr[(ARRAY_SIZE + 1) / 2];
int get_4bits(const unsigned char *arr, size_t index) {
return arr[index >> 1] >> ((index & 1) << 2);
}
int set_4bits(unsigned char *arr, size_t index, int value) {
arr[index >> 1] &= ~ 0x0F << ((index & 1) << 2);
arr[index >> 1] |= (value & 0x0F) << ((index & 1) << 2);
}

Answer 3

You can write your own class for this case. For example:
template <typename T, size_t ITEM_BIT_SIZE>
class BitArrayView {
private:
static const size_t ARRAY_ENTRY_BITS = sizeof(T) * 8;
static const T ITEM_MASK = (~((T) 0)) >> (ARRAY_ENTRY_BITS - ITEM_BIT_SIZE);
T* arr;
public:
struct ItemMutator {
BitArrayView* owner;
size_t index;
T operator=(T value) {
return owner->set(index, value);
}
operator T() {
return owner->get(index);
}
};
const size_t bitSize;
BitArrayView(T* arr, size_t length) : arr(arr), bitSize((length * ARRAY_ENTRY_BITS) / ITEM_BIT_SIZE) {}
T get(size_t index) const {
size_t bitPos = index * ITEM_BIT_SIZE;
size_t arrIndex = bitPos / ARRAY_ENTRY_BITS;
size_t shiftCount = bitPos % ARRAY_ENTRY_BITS;
return (arr[arrIndex] >> shiftCount) & ITEM_MASK;
}
T set(size_t index, T value) {
size_t bitPos = index * ITEM_BIT_SIZE;
size_t arrIndex = bitPos / ARRAY_ENTRY_BITS;
size_t shiftCount = bitPos % ARRAY_ENTRY_BITS;
value &= ITEM_MASK; // trim
arr[arrIndex] &= ~(ITEM_MASK << shiftCount); // clear target bits
arr[arrIndex] |= value << shiftCount; // insert new bits
return value;
}
ItemMutator operator[](size_t index) {
return { this, index };
}
};
And then you may access it like a "bit field" array:
// create array of some uints
unsigned int arr[5] = { 0, 0, 0, 0, 0 };
// set BitArrayView of 3-bit entries on some part of the array
// (two indexes starting at 1)
BitArrayView<unsigned int, 3> arrView(arr + 1, 2);
// should equal 21 now => (2 * 32) / 3
arrView.bitSize == 21;
for (unsigned int i = 0; i < arrView.bitSize; i++) {
arrView[i] = 7; // eg.: 0b111;
}
// now arr[1] should have all bits set
// and arr[2] should have all bits set but last one unset => (2 * 32) % 3 = 1
// the remaining arr items should stay untouched
This is simple implementation which should work with unsigned backing arrays only.
Notice "the mutator trick" in operator[] ;).
Of course some other operators could be implemented, too.

Answer 4

No, bitfields only support integral types. But for very small arrays, you can store each element as a property individually, for example:
struct st
{
unsigned int i0: 1;
unsigned int i1: 1;
unsigned int i2: 1;
unsigned int i3: 1;
unsigned int i4: 1;
};
The disadvantage of this approach is obviously that you can no longer use array-based operations or methods, such as run-time indexing, but it works well enough for basic applications like mathematical vectors.