from_json for non default constructible class with dependency injection

[rvishna]

I have a class without a non default constructor like this:

class SomeStruct
{
public:
    SomeStruct() = delete;
    SomeStruct(std::shared_ptr<ContextData) : contextData_(contextData) { }
private:
    std::shared_ptr<ContextData> contextData_;
}

ContextData is a class that encapsulates dependencies, i.e., data I wish to shared between classes. In my main(), I would create a new ContextData object based on data from, say, a configuration file, and pass it around to classes that need this data.

Is there a way to implement from_json() for SomeStruct? I saw the example in the demo for a move constructible type, but I don't understand how I can capture additional data needed by the constructor of my class that is not coming from the json object argument to from_json().

[nlohmann]

Your type needs to be move-constructible. Then you can follow the example from the documentation: /~https://github.com/nlohmann/json#how-can-i-use-get-for-non-default-constructiblenon-copyable-types

[nlohmann]

Do you need further assistance with this issue?

[rvishna]

I conclude that for a struct to be json serializable, it must be default-constructible or move-constructible, which implies that the only way to achieve dependency injection is to do:

struct SomeStruct
{
    std::shared_ptr<DataStruct> dependencies_;
    void setData(std::shared_ptr<DataStruct> dependencies) 
    {
        dependencies_ = dependencies;
    }
};

instead of this:

struct SomeStruct
{
    std::shared_ptr<DataStruct> dependencies_;
    SomeStruct(std::shared_ptr<DataStruct> dependencies)
    : dependencies_(dependencies)
    {
    }
};

Is this correct? This is what I'm doing now. The downside is that between when the object of SomeStruct is constructed and when the client code decides to call setData(), some members of SomeStruct would have unassigned values. If this is the only way to achieve to_json() and from_json() of SomeStruct, I am happy to have the issue closed.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.