C++ class to access bytes/words of an unsigned integer

Question

union LowLevelNumber
{
 unsigned int n;
 struct
 {
  unsigned int lowByte : 8;
  unsigned int highByte : 8;
  unsigned int upperLowByte : 8;
  unsigned int upperHighByte : 8;
 } bytes;
 struct
 {
  unsigned int lowWord : 16;
  unsigned int highWord : 16;
 } words;     
};

This union allows me to access the unsigned integer byte or word-wise. However, the code looks rather ugly:

var.words.lowWord = 0x66;

Is there a way which would allow me to write code like this:

var.lowWord = 0x66;

Update:
This is really about writing short / beautiful code as in the example above. The union solution itself does work, I just don't want to write .words or .bytes everytime I access lowWord or lowByte.

Answer 1

You could easily wrap that in a class and use get/set accessors.

Answer 2

C++

Would http://www.cplusplus.com/reference/stl/bitset/ serve for your needs?

Plain C version would look something like this:

int32 foo;

//...

//Set to 0x66 at the low byte
foo &= 0xffffff00;
foo |= 0x66;

This is probably going to be more maintainable down the road than writing a custom class/union, because it follows the typical C idiom.

Answer 3

union LowLevelNumber {
    unsigned int n;
    struct {
        unsigned int lowByte : 8;
        unsigned int highByte : 8;
        unsigned int upperLowByte : 8;
        unsigned int upperHighByte : 8;
    };
    struct {
        unsigned int lowWord : 16;
        unsigned int highWord : 16;
    };
};

Note the removed bytes and words names.

Answer 4

You can make

short& loword() { return (short&)(*(void*)&m_source); }

and use it if you don't care parenthesis.

Or you can go fancy

public class lowordaccess
{
   unsigned int m_source;
public:
   void assign(unsigned int& source) { m_source = source; }
   short& operator=(short& value) { ... set m_source }
   operator short() { return m_source & 0xFF; }
}

and then

struct LowLevelNumber
{
   LowLevelNumber() { loword.assign(number); }

   unsigned int number;
   lowordaccess loword;
}
var.loword = 1;
short n = var.loword;

The latter technique is a known property emulation in C++.

Answer 5

Using a union for this is bad, because it is not portable w.r.t. endianness.

Use accessor functions and implement them with bit masks and shifts.