Bit field vs Bitset

Question

I want to store bits in an array (like structure). So I can follow either of the following two approaches

Approach number 1 (AN 1)

struct BIT
{
   int data : 1
};

int main()
{
   BIT a[100];
   return 0;
}

Approach number 2 (AN 2)

int main()
{
    std::bitset<100> BITS;
    return 0;
}

Why would someone prefer AN 2 over AN 1?

Answer 1

Because approach nr. 2 actually uses 100 bits of storage, plus some very minor (constant) overhead, while nr. 1 typically uses four bytes of storage per Bit structure. In general, a struct is at least one byte large per the C++ standard.

#include <bitset>
#include <iostream>

struct Bit { int data : 1; };

int main()
{
    Bit a[100];
    std::bitset<100> b;
    std::cout << sizeof(a) << "\n";
    std::cout << sizeof(b) << "\n";
}

prints

400
16

Apart from this, bitset wraps your bit array in a nice object representation with many useful operations.

Answer 2

A good choice depends on how you're going to use the bits.

std::bitset<N> is of fixed size. Visual C++ 10.0 is non-conforming wrt. to constructors; in general you have to provide a workaround. This was, ironically, due to what Microsoft thought was a bug-fix -- they introduced a constructor taking int argument, as I recall.

std::vector<bool> is optimized in much the same way as std::bitset. Cost: indexing doesn't directly provide a reference (there are no references to individual bits in C++), but instead returns a proxy object -- which isn't something you notice until you try to use it as a reference. Advantage: minimal storage, and the vector can be resized as required.

Simply using e.g. unsigned is also an option, if you're going to deal with a small number of bits (in practice, 32 or less, although the formal guarantee is just 16 bits).

Finally, ALL UPPERCASE identifiers are by convention (except Microsoft) reserved for macros, in order to reduce the probability of name collisions. It's therefore a good idea to not use ALL UPPERCASE identifiers for anything else than macros. And to always use ALL UPPERCASE identifiers for macros (this also makes it easier to recognize them).

Cheers & hth.,