What is the performance of std::bitset?

Question

I recently asked a question on Programmers regarding reasons to use manual bit manipulation of primitive types over std::bitset.

From that discussion I

Answer 1

In addition to what the other answers said about the performance of access, there may also be a significant space overhead: Typical bitset<> implementations simply use the longest integer type to back their bits. Thus, the following code

#include 
#include 

struct Bitfield {
    unsigned char a:1, b:1, c:1, d:1, e:1, f:1, g:1, h:1;
};

struct Bitset {
    std::bitset<8> bits;
};

int main() {
    printf("sizeof(Bitfield) = %zd\n", sizeof(Bitfield));
    printf("sizeof(Bitset) = %zd\n", sizeof(Bitset));
    printf("sizeof(std::bitset<1>) = %zd\n", sizeof(std::bitset<1>));
}

produces the following output on my machine:

sizeof(Bitfield) = 1
sizeof(Bitset) = 8
sizeof(std::bitset<1>) = 8

As you see, my compiler allocates a whopping 64 bits to store a single one, with the bitfield approach, I only need to round up to eight bits.

This factor eight in space usage can become important if you have a lot of small bitsets.