Bug Series - 无效的int变成了有效的unsigned char

Today, i came across this kind of bug:

int iIndex = -1 ;// -1 means invaluable value. >=0 means valuable.

unsigned char ucIdx = iIndex;

if (ucIdx >= 0) { // this is error-prone, most of time ucIdx >= 0 is true.

do sth here;

}

When iIndex = -1, the ucIdx is 255, so even you think iIndex is invaluable, but the ucIdx >= 0 is ture, so error happens.

Why?

Take a two bits value as an example. 2 bits can express 4 values,

unsigned: 00, 01, 10, 11

0, 1, 2, 3.

signed: 11, 00, 01, 10

-1, 0, 1, 2.

It is clear that the signed value -1 means 3 at unsigned value.

so, the ucIdx = iIndex = -1 means 255, which obviously/obviously >= 0. The bug comes from here.