[Boost-bugs] [Boost C++ Libraries] #11907: Why does BOOST_TEST() treat std::string as a collection?

Subject: [Boost-bugs] [Boost C++ Libraries] #11907: Why does BOOST_TEST() treat std::string as a collection?
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-01-12 22:00:39

#11907: Why does BOOST_TEST() treat std::string as a collection?
------------------------------+---------------------
 Reporter:  bspencer@…        |      Owner:  rogeeff
     Type:  Bugs              |     Status:  new
Milestone:  To Be Determined  |  Component:  test
  Version:  Boost 1.59.0      |   Severity:  Problem
 Keywords:                    |
------------------------------+---------------------
 Captured from http://lists.boost.org/boost-users/2015/12/85472.php


 Why does BOOST_TEST() compare std::string values as collections instead of
 scalars (by default)? It's odd that the following produce different
 output, for example:

 {{{
 BOOST_TEST(std::string("a") == "b");
 BOOST_TEST(std::string("a") == std::string("b"));
 }}}

 This will output:

 {{{
 test.cc(6): error: in "test": check std::string("a") == "b" has failed [a
 != b]
 test.cc(7): error: in "test": check std::string("a") == std::string("b")
 has failed
 }}}

 Note the lack of "[a != b]" on the second case. Also note that
 BOOST_CHECK_EQUAL() would emit as in the first form. This seems to be
 because std::string is_forward_iterable, and thus falls
 into the collection_comparison_op.hpp comparators instead of the scalar
 comparators. Is this intentional or an oversight?

 As a simple fix, adding a condition to the partial specialization of the
 collection_comparison_op.hpp comparators that specifically excludes
 std::string is enough to have std::string treated as scalars. For example,
 to handle std::string but not std::wstring with C++11 support:

 {{{
 template<typename Lhs,typename Rhs>
 struct name<Lhs,Rhs,typename boost::enable_if_c<
      unit_test::is_forward_iterable<Lhs>::value &&
      unit_test::is_forward_iterable<Rhs>::value &&
      !(std::is_same<typename std::decay<Lhs>::type, std::string>::value
       || std::is_same<typename std::decay<Rhs>::type, std::string>::value)
 >::type
 }}}

 (The above is an example and not a complete change supporting std::wstring
 and C++98.)

 The response on the mailing list was that this was a bug, so here's a
 ticket to help track it :)

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11907>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.