C ++联合/结构位域实现和可移植性

Question

I have a union containing a uint16 and a struct like so:

union pData {
    uint16_t w1;
    struct {
        uint8_t d1 : 8;
        uint8_t d2 : 4;
        bool b1 : 1;
        bool b2 : 1;
        bool b3 : 1;
        bool b4 : 1;
    } bits;
};

My colleague says there is an issue with this being portable, but I am not sure I buy this. Could some please explain (simple as possible) what is "wrong" here?

Answer 1

From C++17 12.2.4 Bit-fields /1 (and C++11 9.6 Bit-fields /1 for that matter, if you want the answer specific to your chosen tags):

Allocation of bit-fields within a class object is implementation-defined. Alignment of bit-fields is implementation-defined. Bit-fields are packed into some addressable allocation unit. [Note: Bit-fields straddle allocation units on some machines and not on others. Bit-fields are assigned right-to-left on some machines, left-to-right on others. - end note]

Reliance on implementation defined behaviour means, by its very nature, non-portable code.

Answer 2

Maybe your colleague guesses that you intend to write to w1 and read from bits or vice versa.

That would be undefined behaviour. In C++ only one member of a union can be active at any one time; writing a member makes it active, and the behaviour of reading an inactive member is undefined.