Thread: understand masking bits

Hi there,

I am not quite understanding why the program described at Question11-1 from (Page 169) is working - I am expecting it not to work. Might somebody please explain why.

The result should 0 after masking after masking the bits in the line 'if ((flags & DIRECT_CONNECT) != 0)', right?


--- output of program ---

% ./question11-1
High speed set
Direct connect set
%

---- snip -----

Code:

#include <iostream>

const int HIGH_SPEED = (1<<7);      /* modem is running fast */
                                       // we are using a hardwired connection
const int DIRECT_CONNECT = (1<<8);

char flags = 0;

int main()
{
  flags |= HIGH_SPEED;          // we are running fast  
  flags |= DIRECT_CONNECT;      // because we are wired together

  if ((flags & HIGH_SPEED) != 0)
    std::cout << "High speed set\n";

  if ((flags & DIRECT_CONNECT) != 0)
    std::cout << "Direct connect set\n";
  return (0);
}

Why? The code turns on the DIRECT_CONNECT bit with the line: flags |= DIRECT_CONNECT; so it will return non-zero when you check for that bit later.

Your bitwise & in the conditional doesn't modify the flags set in any way. The expression returns a temporary boolean variable that's evaluated (as true in this case) and then disappears. Since flags is never changed, it SHOULD report exactly what you see as the output.