Re: Boost.Test failing to compare std::basic_string<char8_t> strings

From: Leo Carreon (lcarreon_at_[hidden])
Date: 2020-03-09 03:58:47

Hi Mateusz,

Here is the information you requested.

When the following test is performed using Boost.Test:

    auto string_1 = u8"sample"s; // variable initialized with std::u8string 
literal
    auto string_2 = u8"sample"sv; // variable initialized with 
std::u8string_view literal
    BOOST_TEST(string_1 == string_2);

The compilation fails with an error message similar to this:

In file included from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/istream:38,
                 from /usr/include/c++/9/sstream:38,
                 from src/test_description.cpp:9:
/usr/include/c++/9/bits/basic_string.h: In instantiation of ‘class 
std::__cxx11::basic_string<const char8_t, std::char_traits<const char8_t>, 
std::allocator<const char8_t> >’:
/usr/local/include/boost/test/tools/cstring_comparison_op.hpp:75:1: 
required from ‘static bool boost::test_tools::assertion::op::EQ<Lhs, Rhs, 
typename 
boost::enable_if_c<(boost::unit_test::is_cstring_comparable<Lhs>::value && 
boost::unit_test::is_cstring_comparable<Rhs>::value)>::type>::eval(const 
Lhs&, const Rhs&) [with Lhs = std::__cxx11::basic_string<char8_t>; Rhs = 
std::basic_string_view<char8_t>]’
/usr/local/include/boost/test/tools/assertion.hpp:354:24:   required from 
‘boost::test_tools::assertion::binary_expr<Lhs, Rhs, OP>::result_type 
boost::test_tools::assertion::binary_expr<Lhs, Rhs, OP>::value() const [with 
LExpr = 
boost::test_tools::assertion::value_expr<std::__cxx11::basic_string<char8_t> 
 >; Rhs = const std::basic_string_view<char8_t>&; OP = 
boost::test_tools::assertion::op::EQ<std::__cxx11::basic_string<char8_t>, 
std::basic_string_view<char8_t>, void>; 
boost::test_tools::assertion::binary_expr<Lhs, Rhs, OP>::result_type = 
boost::test_tools::assertion_result]’
/usr/local/include/boost/test/tools/assertion.hpp:363:42:   required from 
‘boost::test_tools::assertion_result 
boost::test_tools::assertion::binary_expr<Lhs, Rhs, OP>::evaluate(bool) 
const [with LExpr = 
boost::test_tools::assertion::value_expr<std::__cxx11::basic_string<char8_t> 
 >; Rhs = const std::basic_string_view<char8_t>&; OP = 
boost::test_tools::assertion::op::EQ<std::__cxx11::basic_string<char8_t>, 
std::basic_string_view<char8_t>, void>]’
src/test_description.cpp:50:2:   required from here

Is there a possible workaround other than doing this:

    BOOST_TEST((string_1 == string_2));

Note that I have provided the following functions to allow the printing of 
std::u8string, std::u8string_view and char8_t*:

    std::ostream& std::operator<<(std::ostream&, std::u8string_view);
    std::ostream& std::operator<<(std::ostream&, const char8_t*);

To allow the printing of the compared values.

BTW, Boost.Test seems to be happy with the following test:

    auto string_1 = u8"sample"s; // this creates a std::u8string variable
    auto string_2 = u8"sample";  // this creates a const char8_t* variable
    BOOST_TEST(string_1 == string_2);

-----Original Message----- 
From: Mateusz Loskot via Boost-users
Sent: Sunday, March 8, 2020 11:51 AM
To: boost-users_at_[hidden]
Cc: Mateusz Loskot
Subject: Re: [Boost-users] Boost.Test failing to compare std::basic_string 
strings

On Sun, 8 Mar 2020 at 01:06, Leo Carreon via Boost-users
<boost-users_at_[hidden]> wrote:
>
> Hi Boost.Users,
>
> I’m using Boost.Test from Boost 1.72.0 and I’m getting a failure when I 
> perform the following test:
>
> auto string_1 = u8”value”s;
> auto string_2 = u8”value”s;
> BOOST_TEST(string_1 == string_2);

If you are getting a failure, tell what is the failure (crystal
spheres not always work ;))

I suspect you are hitting two issues:
1. std::u8string is not "eligible for string comparison"
     https://www.boost.org/libs/test/doc/html/boost_test/testing_tools/extended_comparison/strings.html
2. reporting of char8_t falls into reporting of user-defined type
     https://www.boost.org/libs/test/doc/html/boost_test/test_output/test_tools_support_for_logging/testing_tool_output_disable.html

A possible hack to work around these two could be

namespace std {
std::ostream& boost_test_print_type(std::ostream& os, char8_t const& c)
{
    // convert c
    return os;
}
}
BOOST_TEST(string_1 == string_2, boost::test_tools::per_element());

> However, the problem goes away when I change the test line to:
>
> BOOST_TEST((string_1 == string_2));
>
> What is the reason why Boost.Test is failing without the double 
> parentheses?

It disables (de)constructing of the expression from macro input, see
https://www.boost.org/libs/test/doc/html/boost_test/testing_tools/internal_details.html
https://www.boost.org/libs/test/doc/html/boost_test/testing_tools/boost_test_universal_macro.html

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
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