[Boost-bugs] [Boost C++ Libraries] #3469: Fuller support for maps to Boost.Foreach

Subject: [Boost-bugs] [Boost C++ Libraries] #3469: Fuller support for maps to Boost.Foreach
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2009-09-20 21:10:48

#3469: Fuller support for maps to Boost.Foreach
--------------------------------------------------------+-------------------
 Reporter:  Chris Purcell <Chris.Purcell.39@…>          |       Owner:  eric_niebler  
     Type:  Feature Requests                            |      Status:  new           
Milestone:  Boost 1.41.0                                |   Component:  foreach       
  Version:  Boost 1.40.0                                |    Severity:  Not Applicable
 Keywords:                                              |  
--------------------------------------------------------+-------------------
 As documented in the manual
 ([http://www.boost.org/doc/libs/1_40_0/doc/html/foreach/pitfalls.html

 "Pitfalls"]), BOOST_FOREACH is a little awkward to use with associative
 containers. I propose extending the mechanism, adding a new macro that
 assigns key and value to separate values. It also generalizes easily to
 other containers of compile-time-iterable types like Boost.Fusion vectors.

 The proposed syntax:

 {{{
 std::map<int, int> my_map;
 BOOST_FOREACH_FIELD((int key)(int value), my_map)
   std::cout << key << " : " << value << "\n";

 std::vector<boost::fusion::vector<int, std::string, int> >
 my_complex_type;
 BOOST_FOREACH_FIELD((int i)(std::string const& j)(int& k),
 my_complex_type)
   std::cout << "line: " << i << ", " << j << ", " << ++k;
 }}}

 As with the current BOOST.FOREACH macro, the variables to assign to can be
 variables declared before the loop and assigned to at each step, variables
 scoped in the loop and copy-constructed at each step, or references scoped
 in the loop and bound at each step, avoiding copying overhead and allowing
 a mutating loop.

 Existing workarounds I am aware of:

    * Declare a pair variable before the loop, and assign to it at each
 iteration. This does not give meaningful names to the two halves of the
 pair, and has no simple extension to a mutating loop as far as I am aware.
    * Typedef the pair type before the loop (no commas in macros), and use
 a (const) reference in the loop. No copying overhead, but again lacks
 meaningful names.
    * Declare two variables before the loop, and use Boost.Tuple's tie to
 assign to them at each iteration. This gives meaningful names, but has
 copying overhead and no simple extension to a mutating loop.

 Attached is an alpha-quality implementation of the idea, without extensive
 performance or cross-platform testing.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3469>
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