Opened 8 years ago

Closed 5 years ago

#3469 closed Feature Requests (wontfix)

Fuller support for maps in Boost.Foreach

Reported by: Chris Purcell <Chris.Purcell.39@…> Owned by: Eric Niebler
Milestone: Boost 1.42.0 Component: foreach
Version: Boost 1.40.0 Severity: Not Applicable
Keywords: Cc: cblp@…


As documented in the manual (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.

Attachments (1)

foreach_field.hpp (1.9 KB) - added by Chris Purcell <Chris.Purcell.39@…> 8 years ago.
Alpha-quality implementation of feature

Download all attachments as: .zip

Change History (6)

Changed 8 years ago by Chris Purcell <Chris.Purcell.39@…>

Attachment: foreach_field.hpp added

Alpha-quality implementation of feature

comment:1 Changed 8 years ago by Chris Purcell <Chris.Purcell.39@…>

Summary: Fuller support for maps to Boost.ForeachFuller support for maps in Boost.Foreach

comment:2 Changed 8 years ago by Eric Niebler

Status: newassigned

comment:3 Changed 8 years ago by Eric Niebler

Milestone: Boost 1.41.0Boost 1.42.0

For additional motivation for this addition, see this thread.

comment:4 Changed 5 years ago by cblp@…

Cc: cblp@… added

comment:5 Changed 5 years ago by Eric Niebler

Resolution: wontfix
Status: assignedclosed

BOOST_FOREACH is now in legacy, bug-fix-only mode. No new features will be added. Users are encouraged to use the new C++11 range-based for loop.

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