Need help with to_json for derived class, keep getting "cannot use operator"

[PcChip]

Hello,

I have a two part question:

1. I'm having trouble accessing the base class from inside the to_json for the derived class, even though I have it set to "friend" everywhere. I got it somewhat working by doing j = (static_cast<Weapon>(weapon)); , is that the expected way to do it?

2. I'm having trouble with to_json from the derived class, every attempt I make throws the error "cannot use operator[]", or "cannot use at()", even though the exact same code works fine outside of the to_json() function!

I have a base class:

class Weapon
{
    public:
    glm::vec3 firingLocation; //updated every frame with new location
    
    friend void to_json(nlohmann::json& j, const Weapon& weapon); //to access private vars
    friend void from_json(const nlohmann::json& j, Weapon& weapon);//to access private vars

    private:
    glm::vec3 defaultFiringLocation; //location of the barrel in the base/bind pose
};  

and a derived class:

class LaserWeapon :private Weapon
{
public:
	float damagePerTick = 0.0f;
	float burstLength = 0.0f;   //in seconds
	float totalDamage = 0.0f; //total damage if burst is fully applied


	friend void to_json(nlohmann::json& j, const Weapon& weapon); //to access private vars
	friend void from_json(const nlohmann::json& j, Weapon& weapon);//to access private vars
	friend void to_json(nlohmann::json& j, const LaserWeapon& weapon); //to access private vars
	friend void from_json(const nlohmann::json& j, LaserWeapon& weapon);//to access private vars
	friend nlohmann::json;
};

I want to write a to_json function that will produce something similar to this output:

{
    "LaserWeapon",
    {
        "WeaponBase",
        {
		"firingLocation",
		{
			"x": 0.0,
			"y": 0.0,
			"z": 0.0
		}
	},
	"LaserInfo",
	{
		"burstLength", 2.0,
		"totalDamage", 10.0		
	}
    }
}

but I keep getting errors when trying j[] , j.at(), j.emplace etc

when inside the to_json function I can't even use j["test"] = 4; or j.at("test") = 4; without it throwing an error (JSON_THROW(type_error::create(304, "cannot use at() with " + std::string(type_name())));), but that same code works fine inside of main() - what am I doing wrong?

just to show you what I'm trying, I would expect this to work based on documentation, but this is where I keep getting runtime errors:

void to_json(nlohmann::json& j, const LaserWeapon& weapon)
{
	j=(static_cast<Weapon>(weapon)); //only way I could get private vars from base class to work
	
        j["LaserInfo"] = { {"damagePerTick", weapon.damagePerTick},
			 {"burstLength", weapon.burstLength},
			 {"totalDamage", weapon.totalDamage}
	                         };
}  

however I get "cannot use operator[] with a string argument with " + std::string(type_name())

Thanks for any help you can provide!

Edit:
also just to save some time, I should have specified that I've already implemented the glm::vec3 part and confirmed that works:

namespace glm
{
  void from_json(const nlohmann::json& j, glm::vec3& v3)
  {
  	j.at("x").get_to(v3.x);
  	j.at("y").get_to(v3.y);
  	j.at("z").get_to(v3.z);
  }

  void to_json(nlohmann::json& j, const glm::vec3& v3)
  {
  	j = nlohmann::json{ {"x", v3.x}, {"y", v3.y}, {"z", v3.z} };
  }
}

[PcChip]

I think I figured out how to write the json file as I wanted it, but I'm still having trouble reading it back in

This is how I'm writing it out:

void to_json(nlohmann::json& j, const LaserWeapon& weapon)
{
	j = nlohmann::json({ "LaserWeapon", 
			{ "WeaponBase", static_cast<Weapon>(weapon) }, 
			{ "LaserInfo", 
			{"totalDamage", weapon.totalDamage},
			{"burstLength", weapon.burstLength},
			} 
		});

}

and it produces this file:

  
[
    "LaserWeapon",
    [
        "WeaponBase",
        [
            "defaultFiringLocation",
            {
                "x": 2.0,
                "y": 0.0,
                "z": 0.0
            }
        ]
    ],
    [
        "LaserInfo",
        [
            "totalDamage",
            0.0
        ],
        [
            "burstLength",
            0.0
        ]
    ]
]

However when I try to read it back in as laserWeapon = json["LaserWeapon"].get<LaserWeapon>();

I get the same error about operator[]

What would be the correct way to read this back in and fill out the LaserWeapon (and base Weapon) classes?

[nickaein]

The code is creating the JSON as arrays instead of nested objects. For instance, the LaserWeapon doesn't have any child object and it's just an element of top-level array.

You can generate nested objects as demonstrated in README: https://github.com/nlohmann/json#json-as-first-class-data-type

Here is a boiled-down example: https://wandbox.org/permlink/LABEaFLHBIUZMjtQ

[PcChip]

thanks for the tips!

This code:

void to_json(nlohmann::json& j, const LaserWeapon& weapon)
{
  j = nlohmann::json{{ "LaserWeapon",
  		                 { "WeaponBase", static_cast<Weapon>(weapon) },
  		                 { "LaserInfo",
  			                {"totalDamage", weapon.totalDamage},
  			                {"burstLength", weapon.burstLength},
  		                 }
                            }
};

produces this:

[
   [
       "LaserWeapon",
       [
           "WeaponBase",
           [
               "defaultFiringLocation",
               {
                   "x": 0.0,
                   "y": 0.0,
                   "z": 0.0
               }
           ]
       ],
       [
           "LaserInfo",
           [
               "totalDamage",
               0.0
           ],
           [
               "burstLength",
               0.0
           ]
       ]
   ]
]

however I'm really struggling to figure out how to read it back into a LaserWeapon, filling out the base class and the derived class's extra variables... I've tried several methods and most give me an exception about using at() or operator[]

[nickaein]

Again, your code is creating nested arrays which would not be easy to handle. For instance, you have to access x with something probably like:

j[0][1][1][1]["x"]

Instead, you should fix the code to construct nested JSON objects.

Here is a demonstration: https://wandbox.org/permlink/mtQMInu3E2FkmocE

[PcChip]

your example on wandbox is perfect, and I think it should be added to the readme/examples to help others

thanks so much for your time!