Re: [boost] [functional] Generic Selector for STL Pair's and Boost.Tuple's

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-11-21 17:13:27

Dean Michael Berris wrote:

> I've also tried going with fusion::at_c in my initial attempt to get
> the correct result type, only that I run into problems with const
> correctness. I'll try my hand at it some other time though.

What's the problem? I doubt it's const correctness. I'm guessing
it's value/reference confusion. Here's a tip: /value_at/ returns
the actual type while /at/ returns a reference or value depending
on the sequence. For example:

      tuple<int&, int> t(i, 123);

What should /at/ and /value_at/ return? Now, how about this:

     mpl::vector_c<123, 456> is;

What should /at/ and /value_at/ return? /at/ always tries to
return an L-value whenever possible to allow mutation (note:
"whenever possible" -- it's not always possible). /at/ also
tries to return a const-reference, again whenever possible,
to avoid copies. It is naive to simply strip the reference
to get the value because there's no way to know the actual
value if the element is a reference to begin with (first
example). That's where value_at helps.

This *is* very tricky and I suggest you don't reinvent the
wheel.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net