#Integer overflow wrap around code

Quite a few people have been objecting to this, since it can result in the generated code not doing what the programmer intended the problem is becoming more noticeable over time, as compilers introduce more sophisticated optimisation techniques which are more likely to exploit the notion. when both numbers are positive), making it hard to detect when there is an actual problem.A discussion of undefined behaviour and compiler optimisation, particularly in regards to signed integer overflow.Ĭ (and C++) compilers are becoming notorious for exploiting the notion of undefined behaviour - the idea that certain things a program might do have no behaviour proscribed by the language standard, and that the compiler can assume the program doesn't do these things when it is generating object code. And while your compiler should warn you about a signed/unsigned mismatch in this case, your compiler will also generate identical warnings for other cases that do not suffer from this problem (e.g. This program is well formed, compiles, and is logically consistent to the eye. Std::cout << '1 is less than -1 ' // this statement executes arithmetic or comparison) has one signed integer and one unsigned integer, the signed integer will usually be converted to an unsigned integer. In C++, if a mathematical operation (e.g. Second, and more insidiously, unexpected behavior can result when you mix signed and unsigned integers. If you want to compile the code anyway, temporarily disable “Treat warnings as errors”. The above code triggers a warning in some compilers, because the compiler detects that the integer literal is out-of-range for the given type. X = -2 // -2 is out of our range, so we get modulo wrap-around

X = -1 // -1 is out of our range, so we get modulo wrap-around Unsigned short x // smallest 2-byte unsigned value possible Let’s take a look at this using 2-byte shorts: #include 256, however, is outside the range, so it wraps around to the value 0. 255 is in range of a 1-byte integer, so 255 is fine. Any number bigger than the largest number representable by the type simply “wraps around” (sometimes called “modulo wrapping”). Here’s another way to think about the same thing. Therefore, we divide 280 by 256, getting 1 remainder 24. 1 greater than the largest number of the type is 256. The number 280 is too big to fit in our 1-byte range of 0 to 255. If an unsigned value is out of range, it is divided by one greater than the largest number of the type, and only the remainder kept. Given that most programmers would consider this overflow, we’ll call this overflow despite C++’s statements to the contrary. This is contrary to general programming consensus that integer overflow encompasses both signed and unsigned use cases ( cite). Oddly, the C++ standard explicitly says “a computation involving unsigned operands can never overflow”. What happens if we try to store the number 280 (which requires 9 bits to represent) in a 1-byte (8-bit) unsigned integer? The answer is overflow. An integer without a sign (an unsigned integer) assumes all values are positive. Consequently, an integer with a sign (a signed integer) can tell the difference between positive and negative.

If a sign is not provided, we assume a number is positive. The following is a simple way to remember the difference: in order to differentiate negative numbers from positive ones, we use a negative sign. New programmers sometimes get signed and unsigned mixed up. Remembering the terms signed and unsigned When no negative numbers are required, unsigned integers are well-suited for networking and systems with little memory, because unsigned integers can store more positive numbers without taking up extra memory.