Tuple extension proposal

From: Thomas Witt (witt_at_[hidden])
Date: 2002-02-20 05:42:00

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The problem:
- -------------

Creating iterators and function objects that select certain fields of a tuple. 
This is a common problem when working with maps. Think iterating over key or 
data values. Think sgi::select1st. Unfortunately std::pair is not a tuple and 
sgi::selectXX does not scale.

Therefore I propose the following extension to tuple:
- -----------------------------------------------------

The basic idea is define a tuple concept and make std::pair model that 
concept.

What is needed (pseudo code):
- -----------------------------

* Additional overloads for get functions so that pair elements can be accessed

std::pair<int, int> pr(1, 1);
boost::get<0>(pr) == pr.first;

* Tuple traits so that tuple properties can be queried

template<class T>
struct tuple_traits
{
   .....

   template <int N>
   struct element
  {
     typedef ..... type;
  };
};

This will give us:
- ------------------

* Select iterator adaptors that work for tuple and pairs. Select iterators
  are read/write depending on base iterator :

std::map<int, std::string> data;

data[0] = "Null";
data[1] = "One";
data[2] = "Two";
data[3] = "Three";

std::copy(make_select_iterator<1>(data.begin()),
          make_select_iterator<1>(data.end()),
          std::ostream_iterator<std::string>(std::cout, " "));

// Produces "Null One Two Three"

* A more general replacement for sgi::selectXXX

typedef boost::tuple<double, double, int> tTuple;

std::vector< tTuple > tpl;

transform(tpl.begin(), tpl.end(), ostream_iterator<int>(cout, " "),
          select<2, tTuple>());

- ---

This is not just a pipe dream. A working prototype for select iterators is 
available and works for g++/intel/metrowerks.

Any comments would be appreciated

Regards

Thomas

- -- 
Dipl.-Ing. Thomas Witt
Institut fuer Verkehrswesen, Eisenbahnbau und -betrieb, Universitaet Hannover
voice: [phone] - 4273, fax: [phone]
http://www.ive.uni-hannover.de
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