[boost] Re: lexicographic: review request?

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2003-08-24 13:05:34

Jan Langer wrote:
> Daniel Frey wrote:
> 
>> The point is, that Jan proposed to add something which has a hidden 
>> overhead and I'm not sure it's a good idea. Instead of nested 
>> if-else-cascades, the user could also write:
>>
>> bool operator<( const person& lhs, const person& rhs )
>> {
>>    return
>>       lhs.lastname != rhs.lastname ? lhs.lastname < rhs.lastname :
>>       lhs.firstname != rhs.firstname ? lhs.firstname < rhs.firstname :
>>       lhs.age < rhs.age;
>> }
>>
>> This scheme is easy to remember, (IMHO) easy to read, doesn't have 
>> nested if's and it has short-circuiting. It's only cost which may be 
>> removed by Jan's proposal is that it removes the redundancy in typing. 
>> A per line-macro could do that without further problems, but I won't use 
> 
> could you write this macro? i dont see a way to end the recursion.

I guess you are thinking about something different than I was. I just 
wanted to write a macro for each of the above lines. Like this:

#define FOO(a,b) ((a)!=(b))?((a)<(b)):

bool operator<( const persion& lhs, const person& rhs )
{
    return
       FOO( lhs.lastname, rhs.lastname )
       FOO( lhs.firstname, rhs.firstname )
       lhs.age < rhs.age;
}

But wait a minute. Thinking about my old proposed macros once more, I 
think the following might also work:

#define B1(a,b) ((a)!=(b))?((a)<(b)):B2
#define B2(a,b) ((a)!=(b))?((a)<(b)):B1
#define BOOST_LEXICOGRAPHIC B1
const bool B1 = false, B2 = false;

Now your syntax is:

bool operator<( const person& lhs, const person& rhs )
{
    return BOOST_LEXICOGRAPHIC
       ( lhs.lastname, rhs.lastname )
       ( lhs.firstname, rhs.firstname )
       ( lhs.age, rhs.age );
}

Without playing dirty tricks on true/false, with short-circuiting but 
without comparators. (Yes, B1 and B2 should be much longer names)

> on the other hand i should also add a section to the documentation to 
> explain the drawbacks of using this class.

Definitely. It's OK if people know about a problem and choose to ignore 
it. It's not OK if they inherit a problem and they don't know why.

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial training, research and technology
Schloß-Rahe-Straße 15, 52072 Aachen, Germany
fon: [phone], fax: [phone]
eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de