c++ - 具有 4 个 6 位成员的结构的 sizeof

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为什么是这段代码:

#include <iostream>
struct S {
    unsigned char a:6, b:6, c:6, d:6;
};
    int main(int argc, char *argv[]) {
    std::cout << sizeof(S);
    return 0;
}

返回 4？它的大小不应该是 4 x 6 = 24b = 3B 吗？相反，这段代码:

struct S { unsigned char a:4, b:4, c:4, d:4; };

返回一个 2，而这个:

struct S { unsigned char a:4, b:4, c:4, d:4, e:4, f:4; };

返回一个 3...

最佳答案

来自 CppReference: Bit Fields :

The following properties of bit fields are implementation-defined:

[...]

Everything about the actual allocation details of bit fields within the class object

For example, on some platforms, bit fields don't straddle bytes, on others they do

是的，它是实现定义的(即由编译器决定)。