Why does `reinterpret_cast` not behave as the C++ standard says?

Question

At the entry of reinterpret_cast, cppref says:

An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type.The resulting value is the same as the value of expression. (since C++11)

However, the following code cannot be compiled (clang 5.0 with -std=c++1z):

enum class A : int {};

int main()
{
    A a{ 0 };
    reinterpret_cast<int>(a); // error : reinterpret_cast from 'A' to 'int' is not allowed
}

Why does reinterpret_cast not behave as the C++ standard says?

That quote doesn't say anything about `reinterpret_cast(a);`. It does say something about `reinterpret_cast(a);` — Disillusioned, Mar 25 '17 at 04:21

score 5 · Accepted Answer · answered Mar 25 '17 at 04:20

5

The type of a is A, not int. The syntax enum class A : int makes int the underlying type of A, which is a special relationship, but not an "is-a" relationship.

(static_cast will perform this conversion.)

answered Mar 25 '17 at 04:20

Potatoswatter

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score 3 · Answer 2 · answered Mar 25 '17 at 04:30

int is the "underlying type", but the enumeration itself is a separate type.

From [dcl.enum]/5:

Each enumeration defines a type that is different from all other types. Each enumeration also has an underlying type. The underlying type can be explicitly specified using an enum-base.

Why does `reinterpret_cast` not behave as the C++ standard says?

2 Answers2