Re: [boost] Re: Small tuple extension

From: Giovanni P. Deretta (lordshoo_at_[hidden])
Date: 2005-04-26 17:22:05

Larry Evans wrote:
> 
> But isn't an enumerator in some enumeration the equivalent of
> an "element tag"?
> 

Yes,

> 
> The enumerator can also be thought of as a name.
>

yes,

> 
> Which can be done with enumerators also.
>

and also yes.

> 
> But again, I don't see why adding a tag is any better than using an
> enumerator.  The only difference between an array (all elements of
> the same type) and a tuple, is that the type of the value as well
> as the value depends on the index.  And if the types are indexed
> by the same enumeration, then you've solve the problem.  In addition,
> a similar scheme can be used for variants (or disjoint sum).  See
> 
> http://cvs.sourceforge.net/viewcvs.py/boost-sandbox/boost-sandbox/libs/indexed_types/test/sum.cpp?view=markup 
> 

As your code demonstrates, enumerators can be used without duplicating 
information, but an enum and map enum->type is required for each tuple. 
This is verbose and should be wrapped in a macro.  Then again, I don't 
like macros a lot :)

In the end,  getting 'by type' has the same expressive power of using 
enumerators, i simply think that it would have a better syntax and it 
would work out of the box with all tuples without the need of a map.

> 
>>
>> BTW, tuples containing many objects of the same type are better seen
>> as containers, thus the indexed get is fine.
> 
> 
> Agreed, but the indexed get works just as well when the argument is
> an enumerator instead of a literal unsigned, and when it's an
> enumerator, the meaning is clearer:
> 
>     t.get<0>();
>     t.get<1>();
> 
> is obviously not as clear as:
> 
>     t.get<first_field>();
>     t.get<second_field>();
> 
> [snip]

Well, i actually believe the first to be clearer.  It is a matter of 
taste though.

--
Giovanni P. Deretta