Is -1u valid c++?

Question

Is for example

size_t x = -1u;

if (x == -1u)
    ...

valid?

If this is valid it would prevent a warning. of course on a 32 bit system x should be 0xffffffff and on a 64 bit system it should be 0xffffffffffffffff.

-Jochen

Answer 1

1u has the type unsigned int. This is then negated using the unary - operator. The behavior is as follows:

The negative of an unsigned quantity is computed by subtracting its value from 2ⁿ, where n is the number of bits in the promoted operand (C++11 5.3.1/8).

-1u is thus guaranteed to give you the largest value representable by unsigned int.

To get the largest value representable by an arbitrary unsigned type, you can cast -1 to that type. For example, for std::size_t, consider static_cast<std::size_t>(-1).

Answer 2

I've always used ~0U for the purpose of "unsigned, all bits on".

Answer 3

Compiler implementation dependant behavior is annoying. You should be able to do this, though:

size_t x = 0;
x--;

if ((x+1) == 0)