unsigned int causes infinite for-loop

Question

There are two loops below. The first one works well while the second one is an infinite loop. Why?

for (unsigned int i=0; i<3; ++i)
{
    std::cout << "i= " << i << std::endl; // this gives proper result
}

for (unsigned int i=3; i>=0; --i)
{
    std::cout << "i= " << i << std::endl; // infinite loop
}

Answer 1

An unsigned int can never be less than 0. That's what makes it unsigned. If you turn on some warning flags, your compiler should tell you about your problem: i >= 0 is always true for an unsigned value.

Clang, for example, required no special flags at all to warn:

example.cpp:5:29: warning: comparison of unsigned expression >= 0 is always true
      [-Wtautological-compare]
    for (unsigned int i=3; i>=0; --i)
                           ~^ ~
1 warning generated.

GCC required -Wextra:

example.cpp: In function ‘int main()’:
example.cpp:5: warning: comparison of unsigned expression >= 0 is always true