Ticket #11907 (closed Bugs: fixed)
Why does BOOST_TEST() treat std::string as a collection?
|Reported by:||bspencer@…||Owned by:||renficiaud|
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 :)