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

We have this in production code where we had to port MIPS code to x86-64

https://codereview.stackexchange.com/questions/54342/template-for-endianness-free-code-data-always-packed-as-big-endian

Works well for us.

It's basically a template without any storage, the template arguments specify the position of the relevant bits.

If you need multiple fields, you put multiple specializations of the template together in a union, together with an array of bytes to provide storage.

The template has overloads for assignment of value and a conversion operator to unsigned for reading the value.

In addition, if the fields are larger than a byte, they are stored in big-endian byte order, which is sometimes useful when implementing cross-platform protocols.

here's a usage example:

union header
{
    unsigned char arr[2];       // space allocation, 2 bytes (16 bits)

    BitFieldMember<0, 4> m1;     // first 4 bits
    BitFieldMember<4, 5> m2;     // The following 5 bits
    BitFieldMember<9, 6> m3;     // The following 6 bits, total 16 bits
};

int main()
{
    header a;
    memset(a.arr, 0, sizeof(a.arr));
    a.m1 = rand();
    a.m3 = a.m1;
    a.m2 = ~a.m1;
    return 0;
}