Check if all the parameters are within a certain range

If we can check if one variable is within some specific range, we can also do the same thing with multiple variables using fold expressions:

template <typename T, typename ... Ts>
bool within(T min, T max, Ts ...ts)
{
 return ((min <= ts && ts <= max) && ...);
}

The expression, (min <= ts && ts <= max), does tell for every value of the parameter pack if it is between min and max (including min and max). We choose the && operator to reduce all the Boolean results to a single one, which is only true if all the individual results are true.

This is how it looks in action:

within( 10, 20, 1, 15, 30); // --> false
within( 10, 20, 11, 12, 13); // --> true
within(5.0, 5.5, 5.1, 5.2, 5.3) // --> true

Interestingly, this function is very versatile because the only requirement it imposes on the types we use is that they are comparable with the <= operator. And this requirement is also fulfilled by std::string, for example:

std::string aaa {"aaa"};
std::string bcd {"bcd"};
std::string def {"def"};
std::string zzz {"zzz"};

within(aaa, zzz,  bcd, def); // --> true
within(aaa, def,  bcd, zzz); // --> false