Boost C++ Libraries: Ticket #5473: [Foreach] non-const rvalue collections are treated as const collections

mimomorin@… — Fri, 15 Apr 2011 00:11:36 GMT

I made patches for foreach.hpp and libs/foreach/test/, but I cannot attach them since the trac system rejects them as spam...

attachment set

mimomorin@… — Fri, 15 Apr 2011 00:12:36 GMT

attachment → test_mutate_rvalue.patch

Add mutate_rvalue.cpp and update Jamfile (a patch for libs/foreach/test/ against trunk).

mimomorin@… — Fri, 15 Apr 2011 01:13:52 GMT

Still fail to attach a patch for foreach.hpp...

The patch (will be attached) does the following:

Renaming functions

is_const_ --> is_nonconst_lvalue_
- Its return value is also changed.
is_rvalue_ --> is_nonconst_rvalue_
- The name of recently added is_rvalue_(T&&, ...) is not changed because it does detect the rvalueness (the ellipsis parameter is no longer needed and was removed).

Introducing a new macro

BOOST_FOREACH_IS_CONST
- This macro returns "true" if the collection is a const lvalue/rvalue.

Hacking codes for BOOST_FOREACH_RUN_TIME_CONST_RVALUE_DETECTION (this might be an ugly hack...)

simple_variant
- To deal with non-const rvalues, non-const get member function is added. This function returns non-const pointer to the (copied) collection.
- Non-const lvalue collections are detected at compile-time, and so the non-const get function is only invoked for rvalues collections.
begin/end, rbegin/rend
- These functions are modified to accept non-const collections.
- Constness of the collection is propagated by means of auto_any_cast. If the collection is const, auto_any_cast returns const simple_variant reference. Otherwise, it returns non-const simple_variant reference.

All the tests (+ mutate_rvalue.cpp) ran successfully with the following compilers (both in C++03 and C++0x modes): gcc-4.3.5, gcc-4.4.5, gcc-4.5.2, gcc-4.6.0, clang (TOT).

status changed; resolution set

Eric Niebler — Fri, 15 Apr 2011 02:10:50 GMT

status new → closed
resolution → wontfix

Please don't spend any more time on this. Mutable iteration over an rvalue collection makes no sense. I consider the fact that BOOST_FOREACH disallows it a feature. I'm surprised C++0x range-for allows it, but I don't that that's reason enough for BOOST_FOREACH to support it.

mimomorin@… — Fri, 15 Apr 2011 02:38:26 GMT

OK, I agreed with you that mutable iteration over rvalue ranges is non-sense.

bernd.loerwald@… — Mon, 05 Nov 2012 13:03:58 GMT

It actually is not non-sense.

Given a class being a facade container to an actual container, one might construct the facade container on the fly:

values_type values() const { return values_type (*this); }

and use it in a manipulating way:

BOOST_FOREACH (elem_type& elem, values())

with values_type having non-const iterators, that iterate over the actual container (which is referenced in values_type).

This will fail, as values() returns an rvalue, thus only const iteration is possible. It would be perfectly valid to modify the elements iterated over with the facade.