Question

std::bitset has a to_string() method for serializing as a char-based string of 1s and 0s. Obviously, this uses a single 8 bit char for each bit in the bitset, making the serialized representation 8 times longer than necessary.
I want to store the bitset in a binary representation to save space. The to_ulong() method is relevant only when there are less than 32 bits in my bitset. I have hundreds.
I’m not sure I want to use memcpy()/std::copy() on the object (address) itself, as that assumes the object is a POD.

The API does not seem to provide a handle to the internal array representation from which I could have taken the address.

I would also like the option to deserialize the bitset from the binary representation.

How can I do this?

Answer 1

This is a possible approach based on explicit creation of an std::vector<unsigned char> by reading/writing one bit at a time…

template<size_t N>
std::vector<unsigned char> bitset_to_bytes(const std::bitset<N>& bs)
{
    std::vector<unsigned char> result((N + 7) >> 3);
    for (int j=0; j<int(N); j++)
        result[j>>3] |= (bs[j] << (j & 7));
    return result;
}

template<size_t N>
std::bitset<N> bitset_from_bytes(const std::vector<unsigned char>& buf)
{
    assert(buf.size() == ((N + 7) >> 3));
    std::bitset<N> result;
    for (int j=0; j<int(N); j++)
        result[j] = ((buf[j>>3] >> (j & 7)) & 1);
    return result;
}

Note that to call the de-serialization template function bitset_from_bytes the bitset size N must be specified in the function call, for example

std::bitset<N> bs1;
...
std::vector<unsigned char> buffer = bitset_to_bytes(bs1);
...
std::bitset<N> bs2 = bitset_from_bytes<N>(buffer);

If you really care about speed one solution that would gain something would be doing a loop unrolling so that the packing is done for example one byte at a time, but even better is just to write your own bitset implementation that doesn’t hide the internal binary representation instead of using std::bitset.