Cant compile with -C++17 and beyond compiler options

[KennethThompson]

See image below. I've added the #include suggested by a previous post, but that does not seem to resolve this.

[nlohmann]

The error message suggests that you try to convert a JSON value to a user-defined type, but there was not conversion function found. I do not know how this relates to the title of the issue. Could you provide more information?

[KennethThompson]

Ok, so a bit more info. I was previously compiling with the -C++14 option and the project was building fine against your json library. I've since tried to switch to -C++17 (and beyond) and I keep running into the error message above. Here is the function that its complaining about:

[nlohmann]

It's hard to debug based on screen shots. Which version of the library are you using? Could you provide a small example demonstrating the issue? Which compiler are you using?

[KennethThompson]

I'm running against HEAD, let me gen up some example code to simplify

[KennethThompson]

#include <stdio.h>
#include "json.hpp"

    

int main(int argc, char **argv)
{
	printf("hello world\n");
    nlohmann::json _j;
    std::string tmp = _j["test"];  // OK
    
        //std::string tempstring;
        try
        {
            std::string tempstring = _j["setting"]; // OK
        }
        catch( std::exception &ex )
        {
            
        }
        
        std::string tempstring2;
        try
        {
            tempstring2 = _j["setting"];  // FAIL
        }
        catch( std::exception &ex )
        {
            
        }
}

[KennethThompson]

Works just fine when compiling std==c++14, breaks when I attempt to go beyond C++14

[nlohmann]

Which compiler are you using?

[KennethThompson]

gcc thats built into codelite

[nlohmann]

Which version is that?

[KennethThompson]

uhh, let me see..

ken@ken-PowerEdge-R420:/git-repos/json$ g++ --version
g++ (Ubuntu 7.3.0-27ubuntu118.04) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ken@ken-PowerEdge-R420:~/git-repos/json$

[nlohmann]

Thank you. (This just took 4 questions to get all the information that was originally asked in the issue template :-/)

[KennethThompson]

my apologies. I've gotten rusty and lazy

[KennethThompson]

In my defense, I did at least read previous posts on C++17 compiling:)

[nlohmann]

With GCC 7.3 on macOS

g++-7 (Homebrew GCC 7.3.0) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

your code compiles without problems with

• -std=c++11
• -std=c++14
• -std=c++17

[KennethThompson]

odd, could I trouble you to check -std=c++1z ?

[nlohmann]

Works as well.

[KennethThompson]

Please give me your complete list of compile flags you are using

[nlohmann]

Same with

g++ (Ubuntu 8.2.0-7ubuntu1) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[nlohmann]

No flags:

g++-7 bug.cpp -I single_include/nlohmann -std=c++11

[KennethThompson]

ken@ken-PowerEdge-R420:~/git-repos/septemmud/testjson$ /usr/bin/g++-7  -c  "/home/ken/git-repos/septemmud/testjson/main.cpp" -std=c++11  -o ./Debug/main.cpp.o -I. -I. -I../../json/src
ken@ken-PowerEdge-R420:~/git-repos/septemmud/testjson$ /usr/bin/g++-7  -c  "/home/ken/git-repos/septemmud/testjson/main.cpp" -std=c++17  -o ./Debug/main.cpp.o -I. -I. -I../../json/src
In file included from /home/ken/git-repos/septemmud/testjson/main.cpp:2:0:
../../json/src/json.hpp: In instantiation of ‘void nlohmann::detail::from_json_fn::call(const BasicJsonType&, T&, nlohmann::detail::priority_tag<0>) const [with BasicJsonType = nlohmann::basic_json<>; T = std::basic_string_view<char>]’:
../../json/src/json.hpp:1128:47:   required from ‘void nlohmann::detail::from_json_fn::operator()(const BasicJsonType&, T&) const [with BasicJsonType = nlohmann::basic_json<>; T = std::basic_string_view<char>]’
../../json/src/json.hpp:1175:30:   required from ‘static void nlohmann::adl_serializer< <template-parameter-1-1>, <template-parameter-1-2> >::from_json(BasicJsonType&&, ValueType&) [with BasicJsonType = const nlohmann::basic_json<>&; ValueType = std::basic_string_view<char>; <template-parameter-1-1> = std::basic_string_view<char>; <template-parameter-1-2> = void]’
../../json/src/json.hpp:3478:45:   required from ‘ValueType nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::get() const [with ValueTypeCV = std::basic_string_view<char>; ValueType = std::basic_string_view<char>; typename std::enable_if<(((! std::is_same<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>, ValueType>::value) && nlohmann::detail::has_from_json<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>, ValueType>::value) && (! nlohmann::detail::has_non_default_from_json<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>, ValueType>::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’
../../json/src/json.hpp:3737:30:   required from ‘nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::operator ValueType() const [with ValueType = std::basic_string_view<char>; typename std::enable_if<(((! std::is_pointer<_Ptr>::value) && (! std::is_same<ValueType, typename StringType::value_type>::value)) && (! std::is_same<ValueType, std::initializer_list<typename StringType::value_type> >::value)), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer]’
/usr/include/c++/7/bits/basic_string.h:1475:14:   required from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(const _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>&]’
/usr/include/c++/7/bits/basic_string.h:791:35:   required from ‘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=(const _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>&]’
/home/ken/git-repos/septemmud/testjson/main.cpp:25:39:   required from here
../../json/src/json.hpp:1119:9: error: static assertion failed: could not find from_json() method in T's namespace
         static_assert(sizeof(BasicJsonType) == 0,
         ^~~~~~~~~~~~~
ken@ken-PowerEdge-R420:~/git-repos/septemmud/testjson$

[KennethThompson]

C++11 works fine, C++17 does not

[theodelrieu]

This is due to #958.

There are multiple basic_string::operator= overloads, only one of them is templated, thus the compiler picks this one up instead of the std::basic_string one, which is chosen in previous C++ standards.
Looking at constructor (6) on cppreference tells that the compiler tries to convert the templated type to a std::string_view.
Note that allowing this conversion would create a dangling std::string_view, so the library will not add a from_json(json, std::string_view&) in any case.

I suggest using j.get<std::string>() or j.get_to(s) instead.

[KennethThompson]

Thanks! I'll use your suggested alternative

[nlohmann]

@CalielOfSeptem Can I close the issue?

[KennethThompson]

Feel free to close this one, thank you.

…

On Wed, Dec 12, 2018 at 4:15 PM Niels Lohmann ***@***.***> wrote: @CalielOfSeptem <https://github.com/CalielOfSeptem> Can I close the issue? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1362 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ADh5jSApU4sGb3I9TnFQJRphR7VdhbXnks5u4XH1gaJpZM4YoFus> .