gnzlbg / range-based-for

Range-based-for

Document number: NXXXX

Date: XXXX-XX-XX

Project: Programming Language C++, Evolution Working Group

Reply-to: Gonzalo Brito Gadeschi [e-mail]

Range-Based For-Loops: allow iterators with different types

I. Introduction

In the draft N4128 - Ranges for the Standard Library, Revision 1 section 3.3.5 Eric Niebler et al. show that by allowing the begin and end iterators of a range to have different types, range-based libraries can be:

• more efficient,
• more powerful, and
• easier to implement.

The current specification of range-based for loops does not accept begin and end iterators with different types. The users of these libraries have to resort to macros similar in spirit to BOOST_FOREACH (see RANGE_FOR macro in the appendix). This does discourage users from experimenting with these libraries resulting in a situation that is not beneficial for the C++ community.

In the standardese section below it is shown that removing this restriction is a minor non-breaking change. This change is the only core language change required to support the Iterables specified in N4128, see section 3.3.9.

II. Standardese

In 6.5.4.1, replace:

{
  auto && __range = range-init;
  for ( auto __begin = begin-expr,
        __end = end-expr;
        __begin != __end;
        ++__begin ) {
    for-range-declaration = *__begin;
    statement
  }
}

with

{
  auto && __range = range-init;
  auto __begin = begin-expr;
  auto __end = end-expr;
  for ( ; __begin != __end; ++__begin ) {
      for-range-declaration = *__begin;
      statement
  }
}

III. Acknowledgements

The standarese was stated as is in N4128. All kudos go to Eric Niebler, Sean Parent, and Andrew Sutton for that proposal, and to Eric Niebler for the range-v3 library.

IV. References

Eric Niebler, Sean Parent, and Andrew Sutton, N4128 - Ranges for the Standard Library, Revision 1.

Eric Niebler's Range-v3: Range algorithms, views, and actions for the Standard Library documentation.

V. Appendix - RANGE_FOR macro

#define RANGES_FOR(VAR_DECL, ...)                                                               \
    if(bool _range_v3_done = false) {}                                                          \
    else for(auto && _range_v3_rng = (__VA_ARGS__); !_range_v3_done;)                           \
        for(auto _range_v3_begin = ranges::begin(_range_v3_rng); !_range_v3_done;               \
                _range_v3_done = true)                                                          \
            for(auto _range_v3_end = ranges::end(_range_v3_rng);                                \
                    !_range_v3_done && _range_v3_begin != _range_v3_end; ++_range_v3_begin)     \
                if(!(_range_v3_done = true)) {}                                                 \
                else for(VAR_DECL = *_range_v3_begin; _range_v3_done; _range_v3_done = false)   \
    /**/