What does this & operator mean here?

Question

I was reading some tutorial about openGL in qt. One of the mouse event slot has this code in it:

if (event->buttons() & Qt::LeftButton) {    
    rota         


        

Answer 1

That will test that the value event=>buttons() has the bit Qt::LeftButton.

The result would be a 0, if it did not have that bit. and a Qt::LeftButton if it did include that bit.

it is a comparison to check the existence of a flag or bit value on a number

0001 == 1
0010 == 2
0011 == 3

1 & 2 == 0 (false)
1 & 3 == 1 (true)
2 & 3 == 2 (true)

essentially it is a match across the two values for their bit values.

  (0001) 
& (0010)
---------
  (0000) //Neither have the same bit

  (0011)
& (0010)
---------
  (0010) //both have bit 2

  (0101)
& (0110)
---------
  (0100) // Both have the 3rd bit

  (0111)
& (0110)
---------
  (0110) // Both have the 2nd and 3rd bit

Boolean values in C languages are 0 for false. and anything non zero is true.

This proved that the 1st and 2nd bit are available in the number 3. however 1 and 2 do not have matching bits.

Look into bitwise operators. to get a better understanding.

http://en.wikipedia.org/wiki/Bitwise_operation