Signed and Unsigned

Unlike Java, C++ integers come in two "flavors": signed and unsigned. Unsigned variables offer twice the range of positive numbers, but cannot store negative numbers. For example, a 32-bit int has a maximum value of 2,147,483,647, while the maximum unsigned int is 4,294,967,295. C++ allows unsigned int to be abbreviated as unsigned.

Since integers use a fixed amount of memory, what happens if you exceed their range? Unsigned numbers will "wrap around". For instance, try this.

unsigned n = 0;
cout << n - 1 << endl; 

As you can see, the output wraps around from zero to the largest possible unsigned value.

Signed Overflow

This is not necessarily the case with signed numbers, however. Overflow and underflow on signed numbers is undefined behavior. Consider this code:

int n = 2147483647; // max size of 32-bit int
cout << "one larger is " << n + 1 << endl;

On many modern compilers (including ours), if you add the compiler flag
-fsanitize=undefined, you will get a runtime error, like that shown here.

If you leave off that flag, most compilers wrap around just as with signed numbers, and it will print this:

This course content is offered under a CC Attribution Non-Commercial license. Content in this course can be considered under this license unless otherwise noted.