Question

if (~mask == 0){...}

I have encountered this thing in one of the .cpp files, and I wonder what is does ~ mean in c/c++?

Answer 1

It’s a tilde and in C++ it means bitwise NOT.

For an eight-bit unsigned integer named mask with the following bit representation:

0010 1100

the value of ~mask is:

1101 0011

Notice how all the bits have been flipped.

For your if condition (~mask == 0) to evaluate to true:

~mask: 0000 0000
 mask: 1111 1111

In such a case, mask has the value 255.

Apply the same logic to integers of different bit-widths and signedness, as appropriate.

^{(Note: In reality, if your system has 32-bit ints, ~mask will be 32-bit even if mask was 8-bit. This is because ~ performs integral promotion. However, I ignore this fact for the above simple examples.)}

---

Here’s the formal definition:

[C++11: 5.3.1/10]: The operand of ˜ shall have integral or unscoped enumeration type; the result is the one’s complement of its operand. Integral promotions are performed. The type of the result is the type of the promoted operand. There is an ambiguity in the unary-expression ˜X(), where X is a class-name or decltype-specifier. The ambiguity is resolved in favor of treating ˜ as a unary complement rather than treating ˜X as referring to a destructor.

As the passage reminds us, do not confuse bitwise NOT for the leading character in the name of a class destructor. It’s interesting that ~ was chosen for destructors; arguably it’s because one could perceive a destructor as the opposite (i.e. logical NOT) of a constructor.