c++ - 使用 : 限制结构大小

Question

为什么需要这段代码？

typedef struct corr_id_{
    unsigned int  size:8;       
    unsigned int  valueType:8;  
    unsigned int  classId:8;   
    unsigned int  reserved:8;    

} CorrId;

我围绕它做了一些调查，发现通过这种方式我们将内存消耗限制在我们需要的范围内。 例如

typedef struct corr_id_new{
    unsigned int  size;       
    unsigned int  valueType;  
    unsigned int  classId;   
    unsigned int  reserved;   

} CorrId_NEW;

typedef struct corr_id_{
    unsigned int  size:8;       
    unsigned int  valueType:8;  
    unsigned int  classId:8;   
    unsigned int  reserved:8;   

} CorrId;

int main(){
CorrId_NEW Obj1;
CorrId     Obj2;

std::cout<<sizeof(Obj1)<<endl;
std::cout<<sizeof(Obj2)<<endl;
}

输出:-

16
4

我想了解此类场景的真实用例？为什么我们不能像这样声明结构，

typedef struct corr_id_new{
    unsigned _int8  size;       
    unsigned _int8  valueType;  
    unsigned _int8  classId;   
    unsigned _int8  reserved;   

} CorrId_NEW;

这与编译器优化有关吗？或者，以这种方式声明结构有什么好处？

Answer 1

I want to understand the real use case of such scenarios?

例如，某些CPU的状态寄存器结构可能是这样的:

为了通过结构来表示它，您可以使用位域:

struct CSR
{
    unsigned N: 1;
    unsigned Z: 1;
    unsigned C: 1;
    unsigned V: 1;
    unsigned  : 20;
    unsigned I: 1;
    unsigned  : 2;
    unsigned M: 5;
};

你可以在这里看到字段不是 8 的倍数，所以你不能使用 int8_t 或类似的东西。