Question

When I want to negate a number of type std::size_t, I usually do -static_cast<int>(number). However, I understand that the number might not fit into an int. So, my question is what is a safe portable way to do this?

Answer 1

There is no safe portable way to do this.

size_t is an unsigned type. There is no guarantee that there is any signed integer type big enough to hold the maximum value of size_t.

If you’re able to assume that the value you’re negating isn’t too big, you can convert it to long long (if your compiler supports it) or long (if it doesn’t):

size_t s = some_value;
long long negative_s = -(long long)s;

If you’re worried about overflow, you can compare the value of s to LLONG_MAX before doing the conversion.