Why is (-1 < a.size()) false, even though std::vector's size is positive?

Please take a look at this simple program:

#include <iostream>
#include <vector>
 
int main() {
    std::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";
}

The program outputs

0
Greater

I am confused by the fact that it outputs Greater despite -1 obviously being less than 0.

I understand that size member function returns an unsigned value, but comparison is still applied to -1 and 0, so what's going on?

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