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

c++, comparison, size-t, stdvector, unsigned

Solution

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

Problem

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?

Original source