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 struct name::value || std::is_same::value) >::type (The above is an example and not a complete change supporting std::wstring and C++98.) -- Brad Spencer