cutting bits in C++

Question

Suppose we have a binary file, that contains 32 bit numbers. Each 32bit number represents an instruction. My question: is it possible to cut this bits into chunks of 6+5+5+16 directly. Something like:

typedef struct _instruction
{
    int op_code : 6;
    int reg_dest : 5;
    int reg_s1 : 5;
    int offset : 16;
} INST, *PINST;

int read_32_bits = read_next_instr();

INST i = (INST)read_32_bit; /* this would cut the bits into chunks*/

Answer 1

Why not create a bit structure like you have but then place it inside a union? I dropped the typedef so it uses C++ style definitions.

struct instruction
{
    int op_code  : 6;
    int reg_dest : 5;
    int reg_s1   : 5;
    int offset   :16;
};

union INST
{
    instruction  a;
    uint32_t     b;
};

You could store/load the 32-bit values using network functions:

INST  i;
i.b = ntohl(value);

And now you can reference the bit fields without typecasting.

if (i.a.op_code == XXX)