What does tilde(~) operator do?

The ~ operator inverts all the bits. So 10000001 becomes 01111110.

The ~ operator in C++ (and other C-like languages like C and Java) performs a bitwise NOT operation - all the 1 bits in the operand are set to 0 and all the 0 bits in the operand are set to 1. In other words, it creates the complement of the original number.

For example:

10101000 11101001 // Original  (Binary for -22,295 in 16-bit two's complement)
01010111 00010110 // ~Original (Binary for  22,294 in 16-bit two's complement)

In your example, ch=~((ch^i)) performs a bitwise NOT on the bitwise XOR of ch and i then assigns the result to ch.

The bitwise NOT operator has an interesting property that when applied on numbers represented by two's complement, it changes the number's sign and then subtracts one (as you can see in the above example).

You may want become familiar with the different operators of the C++ language since it is difficult to search for operators on search engines. Better yet, you can get a good C++ book which will tell you about the C++ operators.

It is the bitwise complement operator. Given the input

010011101

returns the output:

101100010