Issue with no matching function for call to 'nlohmann::json_abi_v3_12_0::basic_json<>::get<.....>'

[fuegi]

Hi,
As one can see I have a matching issue.

I have defined the following types within a Namespace (hpp file)

namespace JsonParse_ns {
:
class SystemConfig {
  std::string title;
  ...
  ...
}
}

Also created the "from_json" for it (cpp file):

namespace JsonParse_ns {
void from_json(const json& j, SystemConfig& s)
    {
        s.set_title( j.at("Title").get<std::string>() );
        ....
    }
}

Both are within the same Namespace, but in different files. The first is in a .hpp file and the implementaiton is in a .cpp file.

When I try to compile it, I get the following error:

no matching function for call to 'nlohmann::json_abi_v3_12_0::basic_json<>::get<JsonParse_ns::SystemConfig>()'

But when I do it with the "NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE( SystemConfig, title, ...." macro it works (compiles), even without changing anything on the "from_json" code.
But as my variables in the Classes are different to the KEYS in the json file. I would like to proceed with "from_json".

The call producing the above error message is the following in the main program:

SystemConfig sc = jsonCon.get< JsonParse_ns::SystemConfig >();

At the moment I am out of ideas what I am doing wrong or miss. But for sure, I do miss something, but what?

Any idea?

Thanks for any hint!
BR
Andy

I have done searches for similar issues, but none I have found fixed my issue.

[gregmarr]

You need to at least have the declaration of that from_json function visible everywhere that you want to use it. It is not enough for it to exist somewhere else in the program.

[fuegi]

@gregmarr Thanks that fixed it! Thanks again!