Ada subtype equivalent in C++

Question

Does C++ offer something similar to Ada\'s subtype to narrow a type?

E.g.:

type Weekday is (Monday, Tuesday, Wednesday, Thursday, Friday,          


        

Answer 1

Range checking has a cost. C++ has a zero cost policy for features: if you want the feature and you should pay a cost for it, you need to be explicit. That being said, mostly you can use some library or write your own.

Also, what do you expect when someone tries to put Sunday to Working_Day? An exception (most likely)? To set it to Monday? To set it to Friday? Invalidate the object? Keep the same value and ignore that (bad idea)?

As an example:

#include 
#include 
using namespace std;

enum class Weekday
{
    Sunday= 0,
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday
};

template 
class RangedAccess
{
    static_assert(max >= min, "Error min > max");
private:
    T t;

    public:
    RangedAccess(const T& value= min)
    {
        *this= value;
    }


    RangedAccess& operator=(const T& newValue)
    {
        if (newValue > max || newValue < min) {
            throw string("Out of range");
        }
        t= newValue;
    }

    operator const T& () const
    { 
        return t; 
    }

    const T& get() const
    { 
        return t; 
    }
};

using Working_Day= RangedAccess;

int main()
{
    Working_Day workday;

    cout << static_cast(workday.get()) << endl; // Prints 1
    try {
        workday= Weekday::Tuesday;
        cout << static_cast(workday.get()) << endl; // Prints 2
        workday= Weekday::Sunday; // Tries to assign Sunday (0), throws
        cout << static_cast(workday.get()) << endl; // Never gets executed

    } catch (string s) {
        cout << "Exception " << s << endl; // Prints "Exception out of range"
    }
    cout << static_cast(workday.get()) << endl; // Prints 2, as the object remained on Tuesday
}

which outputs:

1
2
Exception Out of range
2