Overflowing of Unsigned Int

c++ integer-overflow unsigned-integer
GILGAMESH picture GILGAMESH · Feb 8, 2012 · Viewed 18.8k times · Source

What will the unsigned int contain when I overflow it? To be specific, I want to do a multiplication with two unsigned ints: what will be in the unsigned int after the multiplication is finished?

unsigned int someint = 253473829*13482018273;

Answer

Pubby picture Pubby · Feb 8, 2012

unsigned numbers can't overflow, but instead wrap around using the properties of modulo.

For instance, when unsigned int is 32 bits, the result would be: (a * b) mod 2^32.

As CharlesBailey pointed out, 253473829*13482018273 may use signed multiplication before being converted, and so you should be explicit about unsigned before the multiplication:

unsigned int someint = 253473829U * 13482018273U;

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