Unable to get_to one particular class

[vulkan101]

Note: solved! See Niels reply - I hadn't used a const json & in from_json.

I'm a bit stumped here. I am just getting started setting up json config export/import for my small test application. So far I have successfully exported and imported a couple of custom data types (my classes), I am aware of the namespace requirement, but I have one class that happens to be the main class for my application which for some reason the from_json is not being recognised.
E.g. my class is called TestApp::App. A class called IO_json has a member _j which is a nlohmann::json,
I can do this:

namespace TestApp {
  void IO_json::save(std::string filename, TestApp::App& app)
  {
    std::ofstream out(filename);
    
    _j["AppConfig"] = json(app);
    out << std::setw(4) << _j << std::endl;
  }
}

and to to_json is called correctly, output looks good.
However I can not compile the following:

void IO_json::load(std::string filename, TestApp::App& app)
 {
   std::ifstream i(filename);
   json j = json::parse(i); 
   
   std::cout << j.dump(4) << '\n';
   
   auto a = j["AppConfig"];
   
   j.at("AppConfig").get_to(app);  // <- this won't compile, 

 }

I get the following:

error C2672: 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to': no matching overloaded function found
1><SNIP)>TestApp\src\io_json.cpp(113,33): error C2783: 'ValueType &nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to(ValueType &) noexcept(<expr>) const': **could not deduce template argument for '__formal'**
1><SNIP>ThirdParty\json\single_include\nlohmann\json.hpp(19589): message : see declaration of 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to'

to and from json looks pretty standard:

namespace TestApp {
void to_json(json& j, const TestApp::App& app)
{
  j = json{ {"camera", app.cam()} };
  auto meshes = app.meshes();    
  j["meshes"] = meshes;
}
void from_json(json& j, TestApp::App& app)
{
  j.at("camera").get_to(app.cam());
}
}

I have quite a few members in my TestApp class; I presumed that I don't need to define conversions for them all just yet? Any ideas where I can look to figure this out?
I'm using the latest develop build and visual studio 2019 Version 16.7.6

[gregmarr]

get_to uses from_json not to_json.

[vulkan101]

did I say that it used to_json? I can't have been clear. The problem is with my trying to compile get_to.
I can use to_json fine as below:
_j["AppConfig"] = json(app);
so that is not the problem.
When I try to use
j.at("AppConfig").get_to(app); // <- this won't compile,
I can't even compile. so there may be an issue with this:

void from_json(json& j, TestApp::App& app)
{
  j.at("camera").get_to(app.cam());
}

but I can't see what it is.
I have implemented this succesfully for a bunch of other classes, but for some reason when trying to get_to for the app class there is an issue.

[nlohmann]

Can you try changing the signature to void from_json(const json& j, TestApp::App& app)

[vulkan101]

wow - facepalm - I can't believe I missed that... yep that fixes it! Nothing mysterious in the class at all, missed the const qualifier for the json object.

[nlohmann]

\o/

[gregmarr]

I only saw the to_json, guess I missed the from_json below it. :) Ah, I saw the get_to line and thought it was the line in main program that wasn't compiling. Sorry for the confusion.