Manage optional values with std::optional

Introduced with C++17, the std::optional class holds an optional value.

Consider the case where you have a function that may or may not return a value – for example, a function that checks if a number is prime but returns the first factor if there is one. This function should return either a value or a bool status. We could create a struct that carries both value and status:

struct factor_t {
    bool is_prime;
    long factor;
};
factor_t factor(long n) {
    factor_t r{};
    for(long i = 2; i <= n / 2; ++i) {
        if (n % i == 0) {
            r.is_prime = false;
            r.factor = i;
            return r;
  &...