Using the spaceship operator.

#include <iostream>
#include <compare>

int main(int argc, char const * argv[]) {
  int a;
  int b;
  std::cout << "value of a? ";
  std::cout.flush();
  std::cin >> a;
  std::cout << "value of b? ";
  std::cout.flush();
  std::cin >> b;
  std::cout << std::endl;

  auto cmp = a <=> b;

  if (std::is_eq(cmp)) {
    std::cout << "a [" << a << "] is equal to b [" << b << ']' << '\n';
  }
  else if (std::is_lt(cmp)) {
    std::cout << "a [" << a << "] is less than b [" << b << ']' << '\n';
  }
  else {
    std::cout << "a [" << a << "] is greater than b [" << b << ']' << '\n';
  }
  std::cout << std::endl;
}