C++ integer overflow

by

Signed integer overflow is undefined behaviour, while unsigned integer overflow is well-defined; the value wraps around. In other words, the value is modulo divided by 2bits, where bits is the number of bits in the data type. Since you’ve a 32-bit int

4294967295 + 1 = 4294967296 % 232 = 0

it results in 0 in the second case. The first case, from the language standpoint, is undefined.

However, most implementations employ 2’s complement to implement signed integer types. A toy, signed 4-bit data type, implemented using 2’s complement may be used to explain what happend in the first case. In this type

POS_MAX = 7 = 0111)2

NEG_MAX = -8 = 1000)2

The type can hold 24 = 16 states, 8 positive (0 to 7) and 8 negative (-1 to -8).

POS_MAX + 1 = 0111)2 + 1)2 = 1000)2

Since the first bit is set, it’s a negative number, to find the actual value, do the inverse the two’s complement (subtract 1 and flip bits)

1000)2 – 1)2 = 0111)2

~0111)2 = 1000)2 = 8

Thus the final value is -8. All this is not defined by the language but this is what happened in your case, specifically.

More Related Contents:

  1. How do I detect unsigned integer overflow?
  2. Why is unsigned integer overflow defined behavior but signed integer overflow isn’t?
  3. Is signed integer overflow still undefined behavior in C++?
  4. Detecting signed overflow in C/C++
  5. (-2147483648> 0) returns true in C++?
  6. Why does long long n = 2000*2000*2000*2000; overflow?
  7. Efficient unsigned-to-signed cast avoiding implementation-defined behavior
  8. Addition of two chars produces int
  9. Overflowing of Unsigned Int
  10. Allowing signed integer overflows in C/C++
  11. How to handle arbitrarily large integers
  12. Program behaving strangely on online IDEs
  13. Checking for underflow/overflow in C++?
  14. Modular Exponentiation for high numbers in C++
  15. How disastrous is integer overflow in C++?
  16. If a 32-bit integer overflows, can we use a 40-bit structure instead of a 64-bit long one?
  17. Is there some meaningful statistical data to justify keeping signed integer arithmetic overflow undefined?
  18. expected ‘}’ at end of input [closed]
  19. Where and why do I have to put the “template” and “typename” keywords?
  20. Example for boost shared_mutex (multiple reads/one write)?
  21. When does a constexpr function get evaluated at compile time?
  22. Why do I get the same sequence for every run with std::random_device with mingw gcc4.8.1?
  23. How to disassemble a binary executable in Linux to get the assembly code?
  24. How do I define a template function within a template class outside of the class definition?
  25. How do I create my own ostream/streambuf?
  26. How do I stop name-mangling of my DLL’s exported function?
  27. Is NaN a valid key value for associative containers?
  28. Branchless code that maps zero, negative, and positive to 0, 1, 2
  29. What’s the motivation behind having copy and direct initialization behave differently?
  30. Is a Linked-List implementation without using pointers possible or not?

Leave a Comment