template <typename T>
class AutoCalcNumber {
  public:
    T& val;
    AutoCalcNumber(T& x) : val(x) {}
    operator T() const {
      return val + 10;
    };
};

long x;
AutoCalcNumber<long> y(x);

void setup() {
  Serial.begin(115200);
  x = 10;
  Serial.println(y); // will print 20
  x = 20;
  Serial.println(y); // will print 30
  long z = y * 2;
  Serial.println(z); // will print 60 but z won't evolve when x evolves. One time evaluation
}

void loop() {}