Question

I’m trying to know the states of different bits over a char

Let’s say I’ve:

char a = 11 //00001011 in binary

How can I retrieve bit number 5 which is currently 0 and cast it to a bool variable? And how can I set it?

Answer 1

As you’ve noted:

char a = 11 // == 00001011 in binary 

Since you said the bit number 5 is clear right now, let’s *define the position of the bits as you’re talking about them:

pos       8  7  6  5  4  3  2  1
value    [0][0][0][0][1][0][1][1]

You want to test the 5th bit, that means you need a mask that accesses the fifth bit, here’s some masks:

your number: 00001011
0x1  mask:   00000001
0x2  mask:   00000010
0x4  mask:   00000100
0x8  mask:   00001000
0x10 mask:   00010000  <-- So that's the one we want, the 5th bit

To set your bit you need a bit-wise OR (|)

a |= 0x10; //   00001011
           // | 00010000  because 1 | anything = 1
           // ----------
           //   00011011  The value is set! 

To test the bit we can use the bit-wise AND (&)

bool something = a & 0x10; //   00001011
                           // & 00010000  because only 1 & 1 = 1
                           // ----------
                           //   00000000  something will be 0 (false) 

---

*it was pointed out that this is a 1-based indexing as opposed to the more typical 0-based indexing. Which is true. Based on the description I chose to read the question as “bit number 5” meaning typical (human) based counting of 1, 2, 3, 4, etc out of 8-bits.

If you’d rather use a 0-based indexing, this is no problem, just “shift” the same logic by one, the mask for the “5th” bit in 0-based indexing is 0x20

When talking about bits it’s good to note which is the least significant bit, and 0 or 1 based to be totally clear.