CODEWITHSUNDEEP
c++
Harry
Harry

Reputation: 2971

enum class of Base class reference as underlying type

I know that the underlying type of an enum class should be an integral. Is it possible to define an enum class with a Base class reference as underlying type?


class CInt {
public:
    CInt() = default;
    virtual ~CInt() = default;

    virtual void fun1() = 0;
    virtual void fun2() = 0;
};

class A : public CInt {
    A() : CInt() {};
    ~A() = default;

    void fun1() {
        std::cout << "A fun1" << std::endl;
    }

    void fun2() {
        std::cout << "A fun2" << std::endl;
    }
};

class B : public CInt {
    B() : CInt() {};
    ~B() = default;

    void fun1() {
        std::cout << "B fun1" << std::endl;
    }

    void fun2() {
        std::cout << "B fun2" << std::endl;
    }
};

// the below code doesn't compile
enum class ClassType : CInt& {
    A_Type, // class A object reference
    B_Type // class B object reference
};

Upvotes: 0

Views: 66

Answers (1)

Adrian Mole
Adrian Mole

Reputation: 51835

The token(s) between the : (if present) and the opening { in a scoped enum declaration specifies what is called – in this Draft C++17 Standard – the type-specifier-seq:

10.2 Enumeration declarations        [dcl.enum]

…
enum-base:
     : type-specifier-seq

Further on in the same section, that type-specifier-seq is constrained as follows:

2   … The type-specifier-seq of an enum-base shall name an integral type; any cv-qualification is ignored. …

Thus, as a reference type is not an integral type, it cannot be used as the "underlying type" of the enum.

From the same Draft Standard, the definition of "integral type" is as follows:

6.7.1 Fundamental types       [basic.fundamental]

…
7     Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types. A synonym for integral type is integer type. …

Note: The equivalent sections in a more-recent Draft Standard can be found here [dcl.enum] and here [basic.fundamental].

Upvotes: 1

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