Re: [Boost] [test] strange behaviour when comparing floating point numbers

From: Edward Diener (eldiener_at_[hidden])
Date: 2020-04-21 20:02:04

On 4/21/2020 3:12 PM, Matthijs Möhlmann via Boost-users wrote:
> Hi,
> 
> I am trying to compare two floating point numbers but am encountering somewhat
> strange behaviour.
> 
> The minimal compilable example:
> #define BOOST_TEST_MODULE tests
> #define BOOST_TEST_DYN_LINK
> #include <boost/test/unit_test.hpp>
> 
> BOOST_AUTO_TEST_SUITE(testsuite)
> 
> BOOST_AUTO_TEST_CASE(test1)
> {
>    namespace tt = boost::test_tools;
>    BOOST_TEST(0.1 == 0.2, tt::tolerance(0.01)); // will fail
>    BOOST_TEST(0.1f == 0.2f, tt::tolerance(0.02)); // will fail too, but different error message...
> }
> 
> BOOST_AUTO_TEST_SUITE_END()
> 
> When compiling with:
> clang++ (clang version 7.0.1-8 (tags/RELEASE_701/final))
> flags: -o main -g -std=c++17 -stdlib=libc++ -lboost_unit_test_framework main.cpp
> boost: 1.72.0
> 
> I'll get the following output:
> Running 1 test case...
> main.cpp(10): error: in "testsuite/test1": check 0.1 == 0.2 has failed [0.10000000000000001 != 0.20000000000000001]. Relative difference exceeds tolerance [1 > 0.01]
> main.cpp(11): error: in "testsuite/test1": check 0.1f == 0.2f has failed [0.100000001 != 0.200000003]
> 
> *** 2 failures are detected in the test module "tests"
> 
> Why am I getting two different messages? I expected those to be the same...

My guess, without knowing how Boost Test works, is that the first checks 
two doubles while the second checks two floats and Boost Test puts out 
different messages for each type.