Is it possible to use array of bit fields?

c++ c arrays struct bit-fields

19,848

Solution 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];

Solution 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);
}

Solution 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.

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