reinterpret_cast error for enum

Question

Why i can't use reinterpret_cast operator for such a cast?

enum Foo { bar, baz };

void foo(Foo)
{
}

int main()
{
   // foo(0); // error: invalid conversion from 'int' to 'Foo'
   // foo(reinterpret_cast<Foo>(0)); // error: invalid cast from type 'int' to type 'Foo'
   foo(static_cast<Foo>(0)); 
   foo((Foo)0);
}

Answer 1

I think that reinterpret_cast can be use for all types of casts, because it's force any type casts to another type with all side-effects of this conversion.

That is a common misconception. Conversions which can be performed with reinterpret_cast are listed explicitly in 5.2.10 of the standard. int-to-enum and enum-to-int conversions are not in the list:

• Pointer to integral type, so long as the integer is large enough to hold it
• nullptr_t to integer
• integral type or enum to pointer
• function pointer to another function pointer of different type
• object pointer to another object pointer of different type
• nullptr_t to other pointer type
• pointer-to-member of T1 to a different pointer-to-member of T2 in cases where both T1 and T2 are objects or functions

reinterpret_cast is typically used to tell the compiler: Hey, I know you think this region of memory is a T, but I'd like you to interpret it as a U (where T and U are unrelated types).

It is also worth noting that reinterpret_cast can have effects on the bits:

5.2.10.3

[ Note: The mapping performed by reinterpret_cast might, or might not, produce a representation dif- ferent from the original value. — end note ]

The C-style cast always works, because it included static_cast in its attempts.

Answer 2

Because regular enum underlying type is int, there is nothing to reinterpret. Static cast is proper conversion for this case.