Is left-shifting a signed integer undefined behavior in C++03?

Question

According to C++03, 5.8/2, left-shifting is defined as follows:

The value of E1 << E2 is E1 (interpreted as a bit pattern) left-shifted E2 bit p

Answer 1

I would like to add that the rules changed in C++11.

In C++11, signed shift left of a negative number is always undefined, even if the underlying machine defines it for values that are in range. It is not implementation-defined, it is undefined. This means that if you do this, the compiler is free to do anything it wants, including delete a bunch of your code unexpectedly. This is in contrast to signed shift right of negative numbers, which is implementation-defined, meaning that its result depends upon the machine type.

Clang's -fsanitize=undefined mode catches attempts to shift left negative numbers.

Answer 2

5.8/2 says that it interprets it as a bit-pattern, which is only implementation dependent if for some reason your implementation doesn't use 2's complement, or if your compiler second-guesses you (they don't). C++11 is more explicit, but says the same thing.

Signed integers use what's known as 2's complement. Basically if you bit-shift a signed integer by 1, if it's positive and below 2^(bits - 2) it will work as if it were unsigned. If it is above that but positive, you will create a strange negative number that bears no relation to the original. If it is negative to begin with, you'll get possibly a negative, possibly a positive number.

For instance, if we have an 8-bit signed integer representing -1:

11111111 // -1

If we left-shift that we end up with

11111110 // -2

However, let's say we have -120

10001000  // -120

We shall end up with

00010000  // 16

Obviously that is not correct!

Continuing, using number 65:

01000001  // 65

Shifted left, this will become:

10000001  // -127

Which equates to -127.

However, the number 16:

00010000 // 16

Shifted left is

00100000 // 32

As you can see, it "sometimes works, sometimes doesn't" but usually works if your number is below 2^(bits-2) and sometimes but not usually if it is above -(2^(bits-2)). That is, to shift left by 1. To shift left by 2, take another bit off. Etc.