[boost] Re: is_enum<enum_UDT const>?

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2003-10-03 17:24:53

On Oct 2, 2003, at 1:46 PM, David Abrahams wrote:

> I note that is_class<X const> is expected to be true iff is_class<X>
> is true.  Does the same rule hold for is_enum?  Is this documented
> somewhere?

It is my understanding that the same holds true for is_enum:

...

enum Enum {zero};

int main()
{
     std::cout << is_enum<Enum>::value << '\n';
     std::cout << is_enum<const Enum>::value << '\n';
     std::cout << is_enum<volatile Enum>::value << '\n';
     std::cout << is_enum<const volatile Enum>::value << '\n';
}

1
1
1
1

I believe that it is implicitly documented but perhaps it could be made 
clearer.  The documents state that for is_enum:

> Evaluates to true only if T is of enum type.

And 3.9.3/1 of the standard says:

> Each type which is a cv-unqualified complete or incomplete object type 
> or is void (basic.types) has three corresponding cv-qualified versions 
> of its type: aconst-qualified version, a volatile-qualified version, 
> and a const-volatile-qualified version.

Therefore a cv-qualified enum is still an enum.

The boost documentation also says:

> For any given type, exactly one of the following expressions will 
> evaluate to true.

So if is_enum<const Enum>::value were to be false, for which primary 
type categorization would it be true?

-Howard