Re: [boost] BOOST_ENUM proposal

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2005-12-07 08:45:04

----Original Message----
From: Frank Laub [mailto:frank.laub_at_[hidden]]
Sent: 07 December 2005 11:47
To: boost_at_[hidden]
Subject: [boost] BOOST_ENUM proposal

> Hello fellow boosters,
> 
> I've recently come up with a way to use the boost preprocesor to
> generate a 'better' enum type. The following is an example of some
> typical usage.  Is anyone else interested in this sort of thing? 

**YES**!!

> I'm
> hoping to get suggestions for how to make it more 'boost-like' in
> both its implementation and its usage. Is this very readable?  Is
> there a place I can upload the code for people to peruse?
> 
> 
> #include <iostream>
> #include <boost/enum.hpp>
> #include <boost/foreach.hpp>
> #define foreach BOOST_FOREACH
I hope that this line is just needed for the demo code in main.

> 
> BOOST_ENUM(Level,
>     (Abort)("unrecoverable problem")
>     (Error)("recoverable problem")
>     (Alert)("unexpected behavior")
>     (Info)("expected behavior")
>     (Trace)("normal flow of execution")
>     (Debug)("detailed object state listings")
> )

Now we come to the meat of the problem.  Here you have defined 'Level' at
directly in a cpp file.  Normally, I would want to define an enum in a
header file, and I see potential problems with violations of the
one-definition-rule.

Also: does your implementation cope with defining an enum inside a class?
(I could live with an answer of "No" - I would just have to put the enum
outside the class.)

Is your implementation limited to 255 elements (normal boost preprocessor
limit)?

Can I specify enum values (ie Abort=0, Info=16)?
> 
> int main()
> {
>     foreach(const string& name, Level::names)
>     {
>         cout << name << endl;
>     }
>     cout << "1: " << Level::names[1] << endl;
>     cout << "toString(Abort): " << Level::toString(Level::Abort) <<
>     endl; cout << "getValue(Abort): " <<
> Level::getValue(Level::Abort) << endl; 
> 
>     Level::type value = Level::Invalid;
>     bool ret = Level::fromString("Abort", value);
>     cout << "fromString(\"Abort\"): " << ret << ", " << value << endl;
> 
>     value = Level::Invalid;
>     ret = Level::fromString("Debug", value);
>     cout << "fromString(\"Debug\"): " << ret << ", " << value << endl;
> 
>     value = Level::Invalid;
>     ret = Level::fromString("not in enum", value);
>     cout << "fromString(\"not in enum\"): " << ret << ", " << value
> << endl; 
> 
>     return 0;
> }
> 
> 
> With the program output being:
> 
> Abort
> Error
> Alert
> Info
> Trace
> Debug
> Invalid
> 1: Error
> toString(Abort): Abort
> getValue(Abort): unrecoverable problem
> fromString("Abort"): 1, 0
> fromString("Debug"): 1, 5
> fromString("not in enum"): 0, 6
> 
> -Frank

-- 
Martin Bonner
Martin.Bonner_at_[hidden]
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: [phone]