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Minimum size implementation for bool array

I need an implementation of the minimum size of a bools array. The size of the array is known at compile time.

I checked std::bitsetand boost::array, but they both carry overhead, which is important for small arrays. For example, if the size of the array is 8 , the container should use only 1 byte of memory (provided that the overall CPU architecture is used).

Is it there, or do I need to roll my own?

EDIT : Here is my final implementation based on Tom Kenapen's post. I added a default value for the constructor and added metalization in the case of an index outside the bounds. Many thanks to Tom and everyone else.

#include <stdexcept>
#include <climits>

/// Minimum size container for bool-arrays
/**
 * TODO: may want to add to_uint32_t accessor and the like
 * for sufficently small arrays
 */
template<int SIZE>
class bitarray
{
public:
    bitarray(bool initial_value = false);

    bool get(int index) const;
    void set(int index, bool value);

private:
    static const int ARRAY_SIZE = (SIZE + CHAR_BIT - 1) / 8;
    unsigned char mBits[ARRAY_SIZE];
};

// ----------------------------------------------------
//      Definitions
// ----------------------------------------------------

template<int SIZE>
inline bitarray<SIZE>::bitarray(bool initial_value)
{
    for(int i = 0; i < ARRAY_SIZE; ++i)
        mBits[i] = initial_value ? -1 : 0;
}

template<int SIZE>
inline bool bitarray<SIZE>::get(int index) const
{
    if (index >= SIZE)
        throw std::out_of_range("index out of range");
    return (mBits[index / CHAR_BIT] & (1 << (index % CHAR_BIT)));
}

template<int SIZE>
inline void bitarray<SIZE>::set(int index, bool value)
{
    if (index >= SIZE)
        throw std::out_of_range("index out of range");
    if (value)
        mBits[index / CHAR_BIT] |= (1 << (index % CHAR_BIT));
    else
        mBits[index / CHAR_BIT] &= ~(1 << (index % CHAR_BIT));
}
+5
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4

. , , , std:: bitset.

#include <climits>
#include <iostream>
#include <cassert>

template<int S> struct boolset {
    static int const SIZE = ((S / CHAR_BIT) + (0 != (S % CHAR_BIT)));
    unsigned char m_bits[SIZE];
public:
    boolset() : m_bits() { for(int i = 0; i < SIZE; ++i) m_bits[i] = 0; }

    bool get(int i) const {
        assert(i < S);
        return (m_bits[i / CHAR_BIT] & (1 << (i % CHAR_BIT)));
    }

    void set(int i, bool v) {
        assert(i < S);
        if(v) { m_bits[i / CHAR_BIT] |= (1 << (i % CHAR_BIT)); }
        else { m_bits[i / CHAR_BIT] &= ~(1 << (i % CHAR_BIT)); }
    }

    void print(std::ostream & s) const {
        for(int i = 0; i < S; ++i) {
            s << get(i);
        }
    }
};

int main(int argc, char ** argv) {
    std::cout << sizeof(boolset<1>) << std::endl;
    std::cout << sizeof(boolset<8>) << std::endl;
    std::cout << sizeof(boolset<9>) << std::endl;
    std::cout << sizeof(boolset<16>) << std::endl;
    std::cout << sizeof(boolset<17>) << std::endl;
    std::cout << sizeof(boolset<32>) << std::endl;
    std::cout << sizeof(boolset<33>) << std::endl;
    std::cout << sizeof(boolset<64>) << std::endl;
    std::cout << sizeof(boolset<129>) << std::endl;
    std::cout << std::endl;
    boolset<31> bs;
    bs.set(0, true);
    bs.set(28, true);
    bs.set(2, true);
    std::cout << bs.get(28) << std::endl;
    bs.print(std::cout); std::cout << std::endl;
    bs.set(2, false);
    bs.print(std::cout); std::cout << std::endl;
}

ideone.

+2

, . bitset , typedef unsigned long _WordT; (SGI) typedef _Uint32t _Ty; (MSVS). . char sizeof 1 (vs2010 pastebin)

+3
template <int N>
class BitSet {
    enum { BPC = 8 }; // Bits per char, #ifdef as needed
    char m_bits[ (N + (BPC-1)) / BPC ];
public:
void SetBit( int i ) { m_bits[ i / BPC ] |= 1 << (i % BPC); }
void ClearBit( int i ) { m_bits[ i / BPC ] &= ~(1 << (i % BPC)); }
int GetBit( int i ) { return (m_bits[ i / BPC ] >> (i % BPC)) & 1; }
};
+2

, - :

#include<vector>
#include <iostream>
template<int N>
struct array
{
   char bits : N;

   int getNthbit(int bitnr)
   {
      // important to make sure bitnr is not larger than the size of the type of `bits` in number of `CHAR_BITS` 
      return bits & (1 << bitnr);
   }
};

//Specialize for N > 8

int main()
{
   std::cout << sizeof(array<8>);
}

If you look at the Live Example , you will see when N == 8 it will return 1 for sizeof(array<8>).

When you put 32, it returns 4.

The only thing you need to do is specialize the pattern for N> 8 so that the type changes according to the number of bits.

I am not a brilliant template, maybe someone needs to write an example?

+1
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