Data structures with different sized bit fields

If I have a requirement to create a data structure that has the following fields:

16-bit Size field

3-bit Version field

1-bit CRC field

How would I c开发者_高级运维ode this struct? I know the Size field would be an unsigned short type, but what about the other two fields?

First, unsigned short isn't guaranteed to be only 16 bits, just at least 16 bits.

You could do this:

struct Data
{
   unsigned short size : 16;
   unsigned char version : 3;
   unsigned char crc : 1;
};

Assuming you want no padding between the fields, you'll have to issue the appropriate instructions to your compiler. With gcc, you can decorate the structure with __attribute__((packed)):

struct Data
{
   // ...
} __attribute__((packed));

In Visual C++, you can use #pragma pack:

#pragma pack(push, 0)
struct Data
{
   // ...
};
#pragma pack(pop)

The following class implements the fields you are looking for as a kind of bitfields.

struct Identifier
{
   unsigned int a; // only bits 0-19 are used
   unsigned int getSize() const {
      return a & 0xFFFF; // access bits 0-15
   }
   unsigned int getVersion() const {
      return (a >> 16) & 7; // access bits 16-18
   }
   unsigned int getCrc() const {
      return (a >> 19) & 1; // access bit 19
   }
   void setSize(unsigned int size) {
      a = a - (a & 0xFFF) + (size & 0xFFF);
   }
   void setVersion(unsigned int version) {
      a = a - (a & (7<<16)) + ((version & 7) << 16);
   }
   void setCrc(unsigned int crc) {
      a = a - (a & (1<<19)) + ((crc & 1) << 19);
   }
};