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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: What's wrong with C++?
Date: Thu, 17 Feb 2011 22:32:48 +0100
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Date: 2011-02-17T22:32:48+01:00
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Hyman Rosen writes on comp.lang.ada:
> On 2/17/2011 3:51 PM, Ludovic Brenta wrote:
>> Hyman Rosen<hyrosen@mail.com>  writes:
>>> C and C++ enumerations don't have arithmetic.
>>> They do convert automatically to integers, but
>>> they don't automatically convert back.
>>
>> Actually they do in the example you provided:
>>
>>      struct DamageType {
>>          enum E { Fire = 1, Acid = 2, Lightning = 4, Poison = 8 };
>>      };
>>
>>      DamageType::E what_my_dragon_can_do =
>>          DamageType::E( DamageType::Acid | DamageType::Poison );
>>
>> In this example, the two enumeration values are automatically promoted
>> to integer (which has arithmetic) ORed, then the result is automatically
>> converted back to the type DamageType::E.
>
> How is that automatic? It is explicitly converted back through
>     DamageType::E( expression )

Because, unless my C++ is really rusty, this is not a conversion.  It is
an explicit call to the (implicitly defined) copy constructor

DamageType::E (DamageType::E&)

where the parameter, of type int, is automatically converted to
DamageType::E&.

An explicit type conversion would look like:

  (DamageType::E) expression

>> Except that the value of what_my_dragon_can_do is not one of the
>> values defined in the type.
>
> Once again, this is a design decision made by those who brought
> enumerations into C. The value an enumeration may take is not
> restricted to being one of the enumerators because the language wants
> to support using enumeration literals as bitmasks. You may not like
> it, but this use is correct C++.

Agreed.  That was my point: C++ is a dangerous language, by design.

-- 
Ludovic Brenta.