Private member initialization with overload of from_json?

[stlamoureux1]

Thanks in advance to all contributors. I'm trying to write overloads for to_json and from_json for my classes so that I can have ordered output and custom formatting, which I don't think I can get with NLOHMANN_DEFINE_TYPE_INTRUSIVE. The problem is I want to assign private member variables when going json -> class, which results in a compiler error.

For now I'm just putting the overloads in the global namespace b/c docs warn against using member methods.

Class declaration (abbreviated)

template <class T>
class Node 
{
public:
    Node() = default;
    Node(unsigned int id, T data);
    ...
private:
    unsigned int m_id;
    T m_data;
}

Overload:

template <class T>
void from_json(const ordered_json& j, Node<T>& node) 
{
    j.at("node").at("id").get_to(node.m_id);
    j.at("node").at("data").get_to(node.m_data);
}

Error msgs:

error: 'm_id' is a private member of 'Node<int>'
    j.at("node").at("id").get_to(node.m_id);
...
error: 'm_data' is a private member of 'Node<int>' j
    .at("node").at("data").get_to(node.m_data);
...

Example Call:

TEST(NodeTest, IOTest3) 
{
    ordered_json j;
    j["node"]["id"] = 0;
    j["node"]["data"] = 0;
    auto node = j.template get<Node<int>>();
    ASSERT_EQ(node.GetId(), 0);
    ASSERT_EQ(node.GetData(), 0);
}

[gregmarr]

In order to access private data directly, you must declare the functions as friends in the same way as the INTRUSIVE macros.

    template<typename BasicJson>
    friend void to_json(BasicJson& j, const Type& t)
    {
    }
    template<typename BasicJson>
    friend void from_json(const BasicJson& j, Type& t)
    {
    }

[stlamoureux1]

Thank you for such a quick response. A couple follow ups if you get the chance:

1. Can you point me to the definition of those macros? I've looked and can't find them.
2. So for this use case, they should be used as member functions? and my Node class would need another template parameter for BasicJson type? (Edit: I think I see what you're saying now -- apologies -- still learning template syntax)

[gregmarr]

1. json/include/nlohmann/detail/macro_scope.hpp

   Line 401 in 960b763

   #define NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type, ...) \

2. This doesn't change how they're used. They're still external to the class, the friend just gives them private access.

[stlamoureux1]

Now I see. Thanks again.