Question

Can somebody clear me out why would I use union and with what purpose the same address for the cin’ed variable and bit field (task from the Schildts C++ book)? In other words why would I use union for :

char ch;
struct byte bit;

// Display the ASCII code in binary for characters.

#include <iostream>
#include <conio.h>
using namespace std;

// a bit field that will be decoded
struct byte {
  unsigned a : 1;
  unsigned b : 1;
  unsigned c : 1;
  unsigned d : 1;
  unsigned e : 1;
  unsigned f : 1;
  unsigned g : 1;
  unsigned h : 1;
};

union bits {
  char ch;
  struct byte bit;
} ascii ;

void disp_bits(bits b);

int main()
{
  do {
    cin >> ascii.ch;
    cout << ": ";
    disp_bits(ascii);
  } while(ascii.ch!='q'); // quit if q typed

  return 0;
}

// Display the bit pattern for each character.
void disp_bits(bits b)
{
  if(b.bit.h) cout << "1 ";
    else cout << "0 ";
  if(b.bit.g) cout << "1 ";
    else cout << "0 ";
  if(b.bit.f) cout << "1 ";
    else cout << "0 ";
  if(b.bit.e) cout << "1 ";
    else cout << "0 ";
  if(b.bit.d) cout << "1 ";
    else cout << "0 ";
  if(b.bit.c) cout << "1 ";
    else cout << "0 ";
  if(b.bit.b) cout << "1 ";
    else cout << "0 ";
  if(b.bit.a) cout << "1 ";
    else cout << "0 ";
  cout << "\n";
}

Answer 1

As a union, both ch and bit have an overlapped (shared) memory location. Store a character in it as ch and then reading bit produces the corresponding bit values for the character.