Fuller support for maps in Boost.Foreach

[Chris Purcell <Chris.Purcell.39@…>]

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.

[Chris Purcell <Chris.Purcell.39@…>]

Alpha-quality implementation of feature

[Eric Niebler]

For additional motivation for this addition, see ​this thread.

[Eric Niebler]

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.