C++17's ambiguous conversion

[Krysme]

In c++ 17, std::string_view can be converted to std::string implicitly which leads to a problem that in c++17 mode , a json object cannot be assigned to a string due to ambiguous assigning conversion path ( json --> string and json --> string_view --> string).

In gcc 7.1 such code produces a compilation error width -std=c++1z.

code:

#include "json.hpp"
#include <string>


int main()
{

    nlohmann::json j;
    j = "12345";
    std::string s;
    s = j;

    return 0;
}

So, is it possible to remove the conversion to string specifically in c++17 mode to let the compiler select the json--> string_view --> string path ?

[nlohmann]

This seems related to #464.

[nlohmann]

Checked with the latest GCC trunk version:

In file included from /Users/niels/Documents/projects/gcc/include/c++/8.0.0/string:52:0,
                 from /Users/niels/Documents/projects/gcc/include/c++/8.0.0/stdexcept:39,
                 from /Users/niels/Documents/projects/gcc/include/c++/8.0.0/array:39,
                 from src/json.hpp:33,
                 from c17.cpp:1:
/Users/niels/Documents/projects/gcc/include/c++/8.0.0/bits/basic_string.h: In instantiation of ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator=(_Tp) [with _Tp = nlohmann::basic_json<>; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> = std::__cxx11::basic_string<char>&]’:
c17.cpp:11:9:   required from here
/Users/niels/Documents/projects/gcc/include/c++/8.0.0/bits/basic_string.h:764:28: error: call of overloaded ‘assign(nlohmann::basic_json<>&)’ is ambiguous
  { return this->assign(__sv); }
                            ^
/Users/niels/Documents/projects/gcc/include/c++/8.0.0/bits/basic_string.h:1313:7: note: candidate: ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
       assign(const basic_string& __str)
       ^~~~~~
/Users/niels/Documents/projects/gcc/include/c++/8.0.0/bits/basic_string.h:1329:7: note: candidate: ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
       assign(basic_string&& __str)
       ^~~~~~
/Users/niels/Documents/projects/gcc/include/c++/8.0.0/bits/basic_string.h:1440:7: note: candidate: ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::__sv_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::__sv_type = std::basic_string_view<char>]’
       assign(__sv_type __sv)
       ^~~~~~

[Krysme]

Thanks for the replying.This error message you posted is exactly what I had.

[nlohmann]

This fixes the issue:

diff --git a/src/json.hpp b/src/json.hpp
index 85d559d7..7bfb7359 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -3742,7 +3742,7 @@ class basic_json
 #ifndef _MSC_VER  // fix for issue #167 operator<< ambiguity under VS2015
                    and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
 #endif
-#if defined(_MSC_VER) && _MSC_VER >1900 && defined(_HAS_CXX17) && _HAS_CXX17 == 1 // fix for issue #464
+#if (defined(__cplusplus) && __cplusplus == 201703L) || (defined(_MSC_VER) && _MSC_VER >1900 && defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
                    and not std::is_same<ValueType, typename std::string_view>::value
 #endif
                    , int >::type = 0 >

[Krysme]

thanks for the fix.
btw is it better to change __cplusplus == 201703L to __cplusplus >= 201703L for future version of c++?

[nlohmann]

Very good point. I shall fix this.