[Boost-users] [test] floating point comparison

Hello all, I've the following test program: #define BOOST_TEST_DYN_LINK #define BOOST_TEST_MAIN #define BOOST_TEST_MODULE Master test suite #include BOOST_AUTO_TEST_CASE(TestCase) { BOOST_REQUIRE_CLOSE_FRACTION(45.00f, 45.17f, 0.004f); } When I compile it with the following line: g++ -o main.o -c -Werror -Wall -std=c++11 -fvisibility=hidden -fvisibility-inlines-hidden -DBOOST_SPIRIT_USE_PHOENIX_V3 -DUTEST -g main.cpp g++ -o main main.o -lboost_unit_test_framework I get the following output: Running 1 test case... *** No errors detected If I lower the tolerance to 0.0004f then I'll get the following: Running 1 test case... main.cpp(8): fatal error in "TestCase": difference{0.00376352} between 45.00f{45} and 45.17f{45.1699982} exceeds 0.00039999999 *** 1 failure detected in test suite "Master test suite" Why does it say that the difference is 0.00376352 and not 0.17....... ? Regards, Matthijs Möhlmann