Re: [boost] [C++0x] Emulation of scoped enums

Subject: Re: [boost] [C++0x] Emulation of scoped enums
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2011-02-16 11:55:52

Vicente Botet <vicente.botet_at_[hidden]> writes:

> Daniel James wrote:
>> 
>> 2009/3/6 vicente.botet <vicente.botet_at_[hidden]>:
>>>
>>> I don't think there is a better solution. You need to mask the fact that
>>> a enum type in C++0x is name scope and a type and can not be in C++03.
>> 
>> A class is a name scope and a type - isn't that why it's called 'enum
>> class'?
>> 
>>     class algae {
>>     public:
>>         enum value {green, red, cyan};
>> 
>>         algae() {}
>>         algae(value v) : v_(v) {}
>> 
>>         // And any other required methods...
>>     private:
>>         value v_;
>>     };
>> 
>>     algae x = algae::red;
>>     algae y = red; // error...
>> 
>> This would also be closer to 'enum class' as it's strongly typed.
>> 
>> Beman's preprocessing syntax would have to change, as the closing
>> macro would have to know the name. The syntax using a preprocessor
>> sequence or varargs would be the same.

This is the approach I've used; I much prefer it to the namespace-based
approach.

> I've worked a little bit with the class approach and it has a main
> portability issue.
>
> The use of user constructors disallows the use of the wrapper class in union
> (c++98 compilers), isn't it?

Yes, but how often do you use unions? I very rarely use unions at all.

> I think that even if we need to use a macro to name the enum class type when
> writing portable code, the namespace approach seems to have less
> limitations.

Having to use a macro whenever I mention the type name defeats the
purpose of using scoped enums --- I might as well write algae::type if I
have to write BOOST_SCOPED_ENUM(algae)

Anthony
-- 
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