求教，c++: struct 和 union 混合使用的问题

[latecho]

最近作业需要模拟解码器，所以采用bit field的思想，构建了如下结构并且进行简单测试(请忽略很多非struct INS_COMP相关的问题)
#include <stdio.h>

struct INS_COMP
{
unsigned int opCode:6;
union
{
struct A
{
union
{
unsigned int rd:5;
unsigned int cond_6_10:5;
};
union
{
unsigned int ra:5;
unsigned int cond_11_15:5;
};
unsigned int rb:5;
union
{
unsigned int imm:11;
unsigned int cond_21_31:11;
};
};
struct B
{
union
{
unsigned int rd:5;
unsigned int cond_6_10:5;
};
union
{
unsigned int ra:5;
unsigned int cond_11_15:5;
};
union
{
unsigned int imm:16;
unsigned int cond_21_31:16;
struct res_cond_imm
{
unsigned int reserve:5;
unsigned int cond:6;
unsigned int imm:5;
};
};
};
};
}I;

int main()
{
std::cout << I.A.imm;
}

出现如下错误:
44 `struct INS_COMP::<anonymous union>::B::<anonymous union>::res_cond_imm' invalid; an anonymous union can only have non-static data members

9 struct INS_COMP::<anonymous union>::A' invalid; an anonymous union can only have non-static data members

28 struct INS_COMP::<anonymous union>::B' invalid; an anonymous union can only have non-static data members

使用Google和百度也搜索了一些文章阅读，但是始终没有像明白，还请高手稍微指点一下。

[Corner]

struct INS_COMP
{
    unsigned int opCode:6;
    union
    {
        struct A
        {
            union
            {
                unsigned int rd:5;
                unsigned int cond_6_10:5;
            };
            union
            {
                unsigned int ra:5;
                unsigned int cond_11_15:5;
            };
            unsigned int rb:5;  
            union
            {  
                unsigned int imm:11;
                unsigned int cond_21_31:11;   
            };
        }A;
        struct B
        {
            union
            {
                unsigned int rd:5;
                unsigned int cond_6_10:5;
            };
            union
            {
                unsigned int ra:5;
                unsigned int cond_11_15:5;
            };
            union
            {
                unsigned int imm:16;
                unsigned int cond_21_31:16;
                struct res_cond_imm
                {
                    unsigned int reserve:5;
                    unsigned int cond:6;
                    unsigned int imm:5;
                };
            };
        }B;
    };
}I;

struct A是类型，所以你的两个结构体成员都没有命名，所以会报错，给他们起个名字就好了

[Corner]

你既然知道给struct INS_COMP变量命名为I，怎么没想到给Struct A的变量命名呢……

struct A
{
union
{
unsigned int rd:5;
unsigned int cond_6_10:5;
};
union
{
unsigned int ra:5;
unsigned int cond_11_15:5;
};
unsigned int rb:5;
union
{
unsigned int imm:11;
unsigned int cond_21_31:11;
};
}A;//增加变量名，避免anonymous

[一叶之舟]

[Quote=引用 1 楼 的回复:]
C/C++ code


struct INS_COMP
{
unsigned int opCode:6;
union
{
struct A
{
union
{
unsigned int rd:5;
uns……
[/Quote]
不要有匿名的成员变量。

[Corner]

你的共用体也是匿名的，共用体在某些编译器下可以匿名，但是不是C99标准，所以最好也有名字，否则遵循C99标准的编译器下上面代码会报错。