What is VC++ doing when packing bitfields?

Question

To clarify my question, let\'s start off with an example program:

#include 

#pragma pack(push,1)
struct cc {
    unsigned int a   :  3;  
            


        

Answer 1

To give another interesting illustrates what's going on, consider the case where you want to pack a structure that crosses a type boundary. E.g.

struct state {
    unsigned int cost     : 24; 
    unsigned int back     : 21; 
    unsigned int a        :  1; 
    unsigned int b        :  1; 
    unsigned int c        :  1;
};

This structure can't be packed into 6 bytes using MSVC as far as I know. However, we can get the desired packing effect by breaking up the first two fields:

struct state_packed {
    unsigned short cost_1   : 16; 
    unsigned char  cost_2   :  8;
    unsigned short back_1   : 16; 
    unsigned char  back_2   :  5;
    unsigned char  a        :  1; 
    unsigned char  b        :  1; 
    unsigned char  c        :  1; 
};

This can indeed be packed into 6 bytes. However, accessing the original cost field is extremely awkward and ugly. One method is to cast a state_packed pointer to a specialized dummy struct:

struct state_cost {
    unsigned int cost     : 24;
    unsigned int junk     :  8; 
};

state_packed    sc;
state_packed *p_sc = ≻

sc.a = 1;
(*(struct state_cost *)p_sc).cost = 12345;
sc.b = 1;

If anyone knows a more elegant way of doing this, I would love to know!