How can I have exactly 2 bits in memory?

Question

I should be able to store a value in a data structure that could go from 0 to 3.. so I need 2 bits. This data structure I will be great 2 ^ 16 locations. So, i want to have 2 ^ 16 * 2 (bits). In C + + do you use to have exactly 2 bits in memory?

Answer 1

You need two bits per unit (not three), so you can pack four units into one byte, or 16 units into one 32-bit integer.

So you will need a std::array<uint32_t, 4096> to accomodate 2¹⁶ units of 2-bit values.

You access the n^th value as follows:

unsigned int get(std::size_t n, std::array<uint32_t, 4096> const & arr)
{
    const uint32_t u = arr[n / 16];
    return (u >> (2 * (n % 16))) & 0x3;
}

Alternatively, you could go with a bitfield:

struct BF32 {
  uint32_t u0 : 2;
  uint32_t u1 : 2;
  //...
  uint32_t uF : 2;
}

And then make an std::array<BF32, 4096>.