User Defined C++11 enum class Default Constructor

Question

Is there a way to specify the default constructor of an enum class?

I am using an enum class to specify a set of values which are allowable for

Answer 1

I know that this question is dated and that it already has an accepted answer but here is a technique that might help in a situation like this with some of the newer features of C++

You can declare this class's variable either non static or static, it can be done in several ways permitted on support of your current compiler.

---

Non Static:

#include 
#include 

template
class PinIDs {
private:
    const std::array ids { IDs... };
public:
    PinIDs() = default;
    const unsigned& operator[]( unsigned idx ) const {
        if ( idx < 0 || idx > ids.size() - 1 ) {
            return -1;
        }
        return ids[idx];
    }
};

---

Static: - There are 3 ways to write this: (First One - C++11 or 14 or higher) last 2 (c++17).

Don't quote me on the C++11 part; I'm not quite sure when variadic templates or parameter packs were first introduced.

template
class PinIDs{
private:        
    static const std::array ids;
public:    
    PinIDs() = default;    
    const unsigned& operator[]( unsigned idx ) const {
        if ( idx < 0 || idx > ids.size() - 1 ) {
            return -1;
        }
        return ids[idx];
    }
};

template
const std::array PinIDs::ids { IDs... };

---

template
class PinIDs{
private:
    static constexpr std::array ids { IDs... }; 
public:   
    PinIDs() = default;    
    const unsigned& operator[]( unsigned idx ) const {
        if ( idx < 0 || idx > ids.size() - 1 ) {
            return -1;
        }
        return ids[idx];
    }
};

---

template
class PinIDs{
private:
    static inline const std::array ids { IDs... };
public:    
    PinIDs() = default;    
    const unsigned& operator[]( unsigned idx ) const {
        if ( idx < 0 || idx > ids.size() - 1 ) {
            return -1;
        }
        return ids[idx];
    }
};

---

All examples above either non-static or static work with the same use case below and provide the correct results:

int main() {
    PinIDs<4, 17, 19> myId;

    std::cout << myId[0] << " ";
    std::cout << myId[1] << " ";
    std::cout << myId[2] << " ";

    std::cout << "\nPress any key and enter to quit." << std::endl;
    char c;
    std::cin >> c;

    return 0;
}

---

Output

4 17 19
Press any key and enter to quit.

---

With this type of class template using a variadic parameter list, you don't have to use any constructor but the default. I did add bounds checking into the array so that the operator[] doesn't exceed bounds of its size; I could of threw an error but with unsigned type I just simply returned -1 as an invalid value.

With this type, there is no default as you have to instantiate this kind of object via template parameter list with a single or set of values. If one wants to they can specialize this class with a single parameter of 0 for a default type. When you instantiate this type of object; it is final as in it can not be changed from its declaration. This is a const object and still holds to be default constructible.