c++ - Why are unsigned integers error prone?

Question

I was looking at this video. Bjarne Stroustrup says that unsigned ints are error prone and lead to bugs. So, you should only use them when you really need them. I've also read in one of the question on Stack Overflow (but I don't remember which one) that using unsigned ints can lead to security bugs.

How do they lead to security bugs? Can someone clearly explain it by giving an suitable example?

Answer 1

One possible aspect is that unsigned integers can lead to somewhat hard-to-spot problems in loops, because the underflow leads to large numbers. I cannot count (even with an unsigned integer!) how many times I made a variant of this bug

for(size_t i = foo.size(); i >= 0; --i)
    ...

Note that, by definition, i >= 0 is always true. (What causes this in the first place is that if i is signed, the compiler will warn about a possible overflow with the size_t of size()).

There are other reasons mentioned Danger – unsigned types used here!, the strongest of which, in my opinion, is the implicit type conversion between signed and unsigned.