Problem with serialization of my custom template doubly-linked list

[blazejwylegly]

Im trying to write a function responsible for saving my custom list into json file.

The list i want to save into file consists of Node objects (Node is inner class of List). The list i need to save uses Doctor objects as T ( List is template class defined in single header file).

Following the guide, I've implemented toJson method for Doctor, and toJson method for my list:
`
template < class T >
void List::toJson(nlohmann::json& j, const List< T >& list) {

j = json::array();
for (Node* current = list.head; current != nullptr; current = current->getNext()) {
	nlohmann::json temp = current->getData();
	j.push_back(temp);
}

}`

(*) getData() method returns reference to Doctor object stored in Node.

Now, in separate class JsonWriter i use method (List.h is included in JsonWriter.h)

void JsonWriter::saveDoctorList(const List< Doctor >& doctorList)
{

file.open("doctorList.json", std::ios::out);
if (file.good()) {
	nlohmann::json myArray = doctorList;
	
}			

}'
...and that's the point where i get stuck. Inside if statement, the compilers signals that
"no suitable user-defined conversion from "const List< Doctor >" to "nlohmann::json" exists".

When i try to do the same nlohmann::json myArray = doctorList; inside my List.h file it compiles without an error.

I've tried to find solution on both github and StackOverflow. I've also looked through issues here, but I can't find anything that solves my problems. Can anybody tell me what am I missing?

Using VS 2017 and Windows

Thanks for any suggestions.

[nlohmann]

You need to implement a free function to_json. See https://github.com/nlohmann/json/blob/develop/README.md#arbitrary-types-conversions for more information.

[blazejwylegly]

Sadly I don't think, that I understand what do you mean by "Free function". I've tried to follow the guide you have linked before posting the issue. I've implemented to_json function for both Doctor and List, similiar to those presented in documentation. I assume, that my List::to_json(...) function is not visible in other files. What am I missing?

[nlohmann]

It must not be a member function.

[blazejwylegly]

I think i've understood in correctly, but im still struggling to solve this. Here's what ive got so far:

• friend void toJson(nlohmann::json& j, const Doctor& obj); declared in Doctor.h file, and definied in Doctor.cpp file.
• friend void toJson(nlohmann::json& j, const List<T>& list); declared in List.h, defined in List.h as following:
  `
  template < class T >

void toJson(nlohmann::json& j, const List< T >& list)
{
j = nlohmann::json::array();
for (List::Node* current = list.head; current != nullptr; current = current->getNext()) {
nlohmann::json temp = current->getData();
j.push_back(temp);
}
};`

• static void saveDoctorList(const List<Doctor>& doc); declared in JsonWriter.h file, defined in JsonWriter.cpp as following:
  `

void JsonWriter::saveDoctorList(const List& doc){
file.open("doctorList.json", std::ios::out);
if (file.good())
{
nlohmann::json obj = doc;
file << obj;
file.close();
}
}
`
Both toJson methods for Doctor and List are defined outside of their class declaration.

Line nlohmann::json obj = doc produces the same error i have mentioned above.
no suitable user-defined conversion from "const List< Doctor >" to "nlohmann::json" exists.

When I try to test same functionality for Doctor in main.cpp
Doctor* doc = new Doctor(...); const Doctor& docRef = *doc;
I get same error:
no suitable user-defined conversion from "const Doctor" to "nlohmann::json" exists.
And that brings me to another question:
Why does line above produce error, and line nlohmann::json temp = current->getData(); from void toJson(nlohmann::json& j, const List< T >& list) compile without an error?

I suppose that my problem is trivial but i can't get over it.

Btw. sorry for poor code pasting.

[nlohmann]

Please have a look at the README. The function must be called to_json just as it is described there.

[blazejwylegly]

Oh... I completely missed it. Didn't think of it at all. Changing function name solved problem. Thank you for your time @nlohmann .