Re: [boost] Enum Conversion

Subject: Re: [boost] Enum Conversion
From: Roland Bock (rbock_at_[hidden])
Date: 2012-09-02 17:36:37

On 2012-09-02 22:46, Vicente J. Botet Escriba wrote:
> Le 02/09/12 21:30, Roland Bock a écrit :
>> Hi,
>>
>> In a lot of projects in the company I work for we use Ice middleware
>> which allows to define nice interfaces for services. The IDL also allows
>> to define enums, but I cannot set specific values for the enum items.
>> We also need to store some of these values in databases. We use int to
>> represent the enumerations.
>>
>> Since we might feel the need to add more items to an enum or delete an
>> unused one, we cannot just cast enum to int and vice versa when writing
>> to or reading from the database. We need methods that translate enum to
>> int and back in a stable way that won't change for an individual enum
>> item, even if its position in the enum changes.
>>
>> I found a way to do this, which provides enum<->int and enum<->string.
>> It is used like this:
>>
>> typedef enum
>> {
>>    Alpha,
>>    Beta,
>>    Gamma,
>>    Delta,
>>    Epsilon
>> } Greek;
>>
>> CREATE_CONVERTER_METHODS(Greek,
>>      (Alpha, 5),
>>      (Beta, 3),
>>      (Gamma, 7),
>>      (Delta, 1),
>>      (Epsilon, 6));
>>
>> std::cout << GreekToString(IntToGreek(1)) << std::endl;
>> std::cout << GreekToString(IntToGreek(6)) << std::endl;
>> std::cout << GreekToInt(Alpha) << std::endl;
>> std::cout << GreekToString(Alpha) << std::endl;
>> std::cout << IntToGreek(17) << std::endl;
>>
>> ------
>>
>> $ ./a.out
>> Delta
>> Epsilon
>> 5
>> Alpha
>> terminate called after throwing an instance of 'std::invalid_argument'
>>    what():  unexpected Greek value
>>
> I've had to manage with these kind of problems very often when working
> with 3pp libraries or 3pp tools.
>> My questions are:
>>
>> a) Is this something that might be interesting for others as well?
>
> I managed defining a specific opaque type, let me say MyGreek, and
> defining implicit conversions from these types.
>
> Your example could be written as
>
> std::cout << string(MyGreek(1)) << std::endl;
> std::cout << string(MyGreek(6)) << std::endl;
> std::cout << MyGreek(Alpha) << std::endl;
> std::cout << MyGreek::Alpha << std::endl;  // using nested literal
> std::cout << string(MyGreek(Alpha)) << std::endl;
> std::cout << string(MyGreek::Alpha) << std::endl; // using nested literal
> std::cout << MyGreek(17) << std::endl;
>
>
> If think this interface corresponds more to how C++ conversions work
> (IMO of course).
>
Thanks for the hints. Implicit conversions might be nicer indeed.

Regards,

Roland