How can I make a variable always equal to the result of some calculations?

Question

In math, if z = x + y / 2, then z will always change whenever we replace the value of x and y. Can we do that in programming

Answer 1

You could write a class that encapsulates its state to update either when mutated or return the right result when requested :

#include 

template
class DynamicCalc
{
public:
    DynamicCalc(const T& func, const U& memberOne, const V& memberTwo) :
        _func(func)
      , _memberOne(memberOne)
      , _memberTwo(memberTwo)
    {

    }

    void SetMemberOne(const U& memberOne) { _memberOne = memberOne; }
    void SetMemberTwo(const U& memberTwo) { _memberTwo = memberTwo; }
    auto Retrieve() { return _func(_memberOne, _memberTwo); }

    U GetMemberOne() { return _memberOne; }
    V GetMemberTwo() { return _memberTwo; }

private: 
    T _func;

    U _memberOne;
    V _memberTwo;
};

int main() {

    auto func = [](int x, int y) {
        return x + y;
    };
    DynamicCalc c(func, 3, 5);

    c.SetMemberOne(5);
    std::cout << c.Retrieve();
}

In truth, if you're happy for the calculation to happen when the value is reuqested then the getters/setters are unnecessary.