no matching overloaded function found when implementing from_json function

[warrenbocphet]

Hi all,

I have been using nlohmann's json library for the last few days on Linux and it works great!

Switched to Windows yesterday to make sure my code can cross-compile and it doesn't! Here's a minimal version to replicate the problem:

#include <nlohmann/json.hpp>

namespace messages
{
using nlohmann::json;

struct JobDetails
{
    std::string config;
    double config_version;
    std::string job_type_id;
};

}  // namespace messages

namespace nlohmann
{
inline void from_json(const json& j, messages::JobDetails& x)
{
    x.config = j.at("config").get<std::string>();
    x.config_version = j.at("configVersion").get<double>();
    x.job_type_id = j.at("jobTypeId").get<std::string>();
}
}  // namespace nlohmann

int main(int argc, char** argv) { return 0; }

I use CMake to compile and VS2019 as my compiler. The library version is 3.10.4. The error I got is the follow:

Microsoft (R) Build Engine version 16.8.3+39993bd9d for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

  Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29337 for x64
  Copyright (C) Microsoft Corporation.  All rights reserved.
  main.cpp
  cl /c /IC:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\include /I 
  C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src /IC:\Users\atr 
  an\Desktop\source_code\template_project_for_testing\install\include /W4 /WX- /diagnostics:c 
  olumn /O2 /Ob2 /D WIN32 /D _WINDOWS /D NDEBUG /D _USE_MATH_DEFINES /D JSON_USE_IMPLICIT_CON 
  VERSIONS=1 /D JSON_DIAGNOSTICS=0 /D "CMAKE_INTDIR=\"Release\"" /D _MBCS /Gm- /EHsc /MD /GS  
  /fp:precise /Zc:wchar_t /Zc:forScope /Zc:inline /GR /std:c++17 /Fo"some_app.dir\Release\\"  
  /Fd"some_app.dir\Release\vc142.pdb" /Gd /TP /errorReport:queue C:\Users\atran\Desktop\sourc 
  e_code\template_project_for_testing\some_app\src\main.cpp
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(30,30):
 error C2672: 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,do
uble,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::g 
et': no matching overloaded function found [C:\Users\atran\Desktop\source_code\template_proje 
ct_for_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(30,48): 
 error C2893: Failed to specialize function template 'unknown-type nlohmann::basic_json<std:: 
map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_seriali 
zer,std::vector<uint8_t,std::allocator<uint8_t>>>::get(void) noexcept(<expr>) const' [C:\User 
s\atran\Desktop\source_code\template_project_for_testing\build\some_app\some_app.vcxproj]     
C:\Users\atran\Desktop\source_code\template_project_for_testing\install\include\nlohmann/json 
.hpp(20708): message : see declaration of 'nlohmann::basic_json<std::map,std::vector,std::str 
ing,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t, 
std::allocator<uint8_t>>>::get' [C:\Users\atran\Desktop\source_code\template_project_for_test 
ing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(30,48): 
 message : With the following template arguments: [C:\Users\atran\Desktop\source_code\templat 
e_project_for_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(30,48): 
 message : 'ValueTypeCV=std::string' [C:\Users\atran\Desktop\source_code\template_project_for 
_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(30,48): 
 message : 'ValueType=std::basic_string<char,std::char_traits<char>,std::allocator<char>>' [C 
:\Users\atran\Desktop\source_code\template_project_for_testing\build\some_app\some_app.vcxpro 
j]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(31,45): 
 error C2672: 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,do 
uble,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::g 
et': no matching overloaded function found [C:\Users\atran\Desktop\source_code\template_proje 
ct_for_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(31,58):
 error C2893: Failed to specialize function template 'unknown-type nlohmann::basic_json<std:: 
map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_seriali 
zer,std::vector<uint8_t,std::allocator<uint8_t>>>::get(void) noexcept(<expr>) const' [C:\User 
s\atran\Desktop\source_code\template_project_for_testing\build\some_app\some_app.vcxproj]     
C:\Users\atran\Desktop\source_code\template_project_for_testing\install\include\nlohmann/json 
.hpp(20708): message : see declaration of 'nlohmann::basic_json<std::map,std::vector,std::str 
ing,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t, 
std::allocator<uint8_t>>>::get' [C:\Users\atran\Desktop\source_code\template_project_for_test 
ing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(31,58): 
 message : With the following template arguments: [C:\Users\atran\Desktop\source_code\templat 
e_project_for_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(31,58): 
 message : 'ValueTypeCV=double' [C:\Users\atran\Desktop\source_code\template_project_for_test 
ing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(31,58): 
 message : 'ValueType=double' [C:\Users\atran\Desktop\source_code\template_project_for_testin 
g\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(32,38): 
 error C2672: 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,do 
uble,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::g 
et': no matching overloaded function found [C:\Users\atran\Desktop\source_code\template_proje 
ct_for_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(32,56): 
 error C2893: Failed to specialize function template 'unknown-type nlohmann::basic_json<std:: 
map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_seriali 
zer,std::vector<uint8_t,std::allocator<uint8_t>>>::get(void) noexcept(<expr>) const' [C:\User 
s\atran\Desktop\source_code\template_project_for_testing\build\some_app\some_app.vcxproj]     
C:\Users\atran\Desktop\source_code\template_project_for_testing\install\include\nlohmann/json 
.hpp(20708): message : see declaration of 'nlohmann::basic_json<std::map,std::vector,std::str 
ing,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t, 
std::allocator<uint8_t>>>::get' [C:\Users\atran\Desktop\source_code\template_project_for_test 
ing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(32,56): 
 message : With the following template arguments: [C:\Users\atran\Desktop\source_code\templat 
e_project_for_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(32,56): 
 message : 'ValueTypeCV=std::string' [C:\Users\atran\Desktop\source_code\template_project_for 
_testing\build\some_app\some_app.vcxproj]
C:\Users\atran\Desktop\source_code\template_project_for_testing\some_app\src\main.cpp(32,56): 
 message : 'ValueType=std::basic_string<char,std::char_traits<char>,std::allocator<char>>' [C 
:\Users\atran\Desktop\source_code\template_project_for_testing\build\some_app\some_app.vcxpro 
j]
CMake Build failed!

Moving the from_json function into the namespace of JobDetails will result in a success in compiling. However, I did not have to do that in Linux? Is there some weird benefit for the library when compiling in Linux?

And in case you ask why I have such a weird code structure, the code is actually being auto generated from a json schema using this website: https://app.quicktype.io/

Thanks in advance!

[nlohmann]

The from_json function must be defined in the same namespace as the type, not nlohmann.

[SylvainFogale]

"The from_json function must be defined in the same namespace as the type, not nlohmann. " => Firstly I did not understand that the code imported from https://app.quicktype.io/ needed to be corrected, because initially it worked fine.

Then this compilation error no "matching overloaded function" for "get" appeared on existing code. I traced down to switching one Visual Studio 2022 project option "language conformance" "permissive" rather than default option (using language C++20). Reverting this option back to default "not permissive" recovered the compilation.

Now I am questioning whether

• I should "trust" the strict conformance of Visual Studio and keep compiling https://app.quicktype.io/ generated code as it is
• make the manual correction in autogenerated code and take the risk that I overwrite it at some point in the future
• I also wonder if a "using namespace nlohmann" somewhere can allow direct compilation of code generated by https://app.quicktype.io/ although this automatic tool does not conform to the author intent ?

I want to get a good understanding of the situation because I plan to rely on this lib all my life !
This lib is so great and the interoperation with https://app.quicktype.io/ makes the combination overly powerfull.
The "to" and "from" beeing not members, the lib is less invasive than solution that gets the member<->json field mapping only once for either way, but that need to be implemented inside the class.

[falbrechtskirchinger]

I suggest you file an issue with quicktype. Their usage of this library is simply incorrect.

[SylvainFogale]

So I understand we should stick to Nlohmann directive, and adjust the generated code. Thank you for the clarification.
The open issue exist in quicktype indeed. See :
glideapps/quicktype#1185 (comment)
glideapps/quicktype#1494
I should have read this first if only I knew. I hope at least your response will help other people to follow the track, circumvent the situation until the automatic code generation is updated. Thank you very much !

[falbrechtskirchinger]

@nlohmann Even worse, they have an option to move the functions into nlohmann::detail.

Moves to_json and from_json types into the nlohmann::details namespace, so that msbuild can build it with conformance mode disabled

[nlohmann]

Yes, this is indeed a problem. Happy to help fixing this, but it's definitely broken on quicktype's end.

[falbrechtskirchinger]

I took a brief look at their issues and PR activity and decided to spend my time more wisely. If anyone can get a hold of a dev to get some assurance that a PR would be looked at in a timely manner, I'd reconsider.

[dvdsgl]

This is fixed: glideapps/quicktype#1996