C++ union/结构位域实现和可移植性

Question

我有一个包含 uint16 和结构的 union :

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;
};

我的同事说这个便携性有问题，但我不确定我是否购买了这个。有人能解释一下(尽可能简单)这里的“错误”是什么吗？

Answer 1

来自 C++17 12.2.4 Bit-fields/1(和 C++11 9.6 Bit-fields/1，如果你想要回答特定您选择的标签):

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]

依赖于实现定义的行为，就其本质而言，意味着不可移植的代码。