Is there a portable alternative to C++ bitfields

Question

There are many situations (especially in low-level programming), where the binary layout of the data is important. For example: hardware/driver manipulation, network protocols,

Answer 1

It's simple to implement bit fields with known positions with C++:

template
struct BitField {
    T *data;

    BitField(T *data) : data(data) {}

    operator int() const {
        return ((*data) >> POS) & ((1ULL << SIZE)-1);
    }

    BitField& operator=(int x) {
        T mask( ((1ULL << SIZE)-1) << POS );
        *data = (*data & ~mask) | ((x << POS) & mask);
        return *this;
    }
};

The above toy implementation allows for example to define a 12-bit field in a unsigned long long variable with

unsigned long long var;

BitField muxno(&var);

and the generated code to access the field value is just

0000000000000020 <_Z6getMuxv>:
  20:   48 8b 05 00 00 00 00    mov    0x0(%rip),%rax  ; Get &var
  27:   48 8b 00                mov    (%rax),%rax     ; Get content
  2a:   48 c1 e8 07             shr    $0x7,%rax       ; >> 7
  2e:   25 ff 0f 00 00          and    $0xfff,%eax     ; keep 12 bits
  33:   c3                      retq   

Basically what you'd have to write by hand