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 can define the following lambda z which always returns the current value of x+y because x and y are captured by reference:

DEMO

int main()
{
    int x;
    int y;

    const auto z = [&x, &y](){ return x+y; };

    std::cin  >> x; // 1
    std::cin  >> y; // 2
    std::cout << z() << std::endl; // 3

    std::cin  >> x; // 3
    std::cin  >> y; // 4
    std::cout << z() << std::endl; // 7
}