c++ vector size. why -1 is greater than zero

Please take a look at this simple program:

#include <iostream>
#include <vector>

using namespace std;

int main() {

vector<int> a;

std::cout << "vector size " << a.size() << std::endl;

int b = -1;

if (b < a.size())
   std::cout << "Less";
else
   std::cout << "Greater";

    return 0;
}

I'm confused by the fact that it outputs "Greater" despite it's obvious that -1 is less than 0. I understand that size method returns unsigned value but comparison is still applied to -1 and 0. So what's going on? can anyone explain this?

Solution 1:

Because the size of a vector is an unsigned integral type. You are comparing an unsigned type with a signed one, and the two's complement negative signed integer is being promoted to unsigned. That corresponds to a large unsigned value.

This code sample shows the same behaviour that you are seeing:

#include <iostream>
int main()
{
  std::cout << std::boolalpha;
  unsigned int a = 0;
  int b = -1;
  std::cout << (b < a) << "\n"; 
}

output:

false

Solution 2:

The signature for vector::size() is:

size_type size() const noexcept;

size_type is an unsigned integral type. When comparing an unsigned and a signed integer, the signed one is promoted to unsigned. Here, -1 is negative so it rolls over, effectively yielding the maximal representable value of the size_type type. Hence it will compare as greater than zero.