Best way to output a vector of a given type to json

[arnaudbrejeon]

This is not a feature request, but more a question.

I'll give you my exact use case so it is easy to understand.
I have a 3D point struct:
struct Point { float x, y, z; };

I have a very large vector of this points and would like to output them to json with each point in the format: { "x": x, "y": y, "z": z}

I can easily do it by creating an array and using emplace_back. It works, but it is slow because the current API doesn't provide a resize or a reserve like std::vector does.

Is there a way to create my json as fast as possible?
Thank you

[nlohmann]

This is described in https://github.com/nlohmann/json#arbitrary-types-conversions: Basically, you need to write a void to_json(json& j, const Point& p) function in the namespace of Point where you set j to your preference, e.g. j = {{"x", p.x}, {"y", p.y}, {"z", p.z}};. Then you can write

Point p {1,2,3};
json j = p;

[arnaudbrejeon]

How do I convert the vector of Point, then? Still with successive emplace_back?

[nlohmann]

Once you define how Point -> json works, then a std:.vector<Point> will be automatically translated to an array.

[arnaudbrejeon]

Oh nice!
Thank you for your quick reply!

[nlohmann]

For reference:

#include <iostream>
#include <iomanip>
#include "json.hpp"

using json = nlohmann::json;

struct Point { float x, y, z; };

void to_json(json& j, const Point& p)
{
    j = {{"x", p.x}, {"y", p.y}, {"z", p.z}};
}

int main(int argc, const char **argv) {
    Point p1 = {1,2,3};
    Point p2 = {3,4,5};
    
    std::vector<Point> v = {p1, p2};
    json j = v;
    
    std::cout << std::setw(2) << j << std::endl;
}

Output:

[
  {
    "x": 1.0,
    "y": 2.0,
    "z": 3.0
  },
  {
    "x": 3.0,
    "y": 4.0,
    "z": 5.0
  }
]
``

[ElHouas]

Thanks for the post really useful,

One question, if this point data comes from a loop and I will like to concatenate it and write it in a file. How can I do it?

The output should look like this:

[
"segment1",
  [
      "point",
      {
        "x": 1,
        "y":2 
      },
      "point",
      {
        "x": 4,
        "y": 5.
      },
      "point",
      {
        "x": 8,
        "y": 3
      },
      "point",
      {
        "x": 2,
        "y": 2
      }
  ]
"segment2",
  [
      "point",
      {
        "x": 1,
        "y": 2
      },
      "point",
      {
        "x": 3,
        "y": 4
      },
      "point",
      {
        "x":5 ,
        "y": 6
      },
      "point",
      {
        "x": 7,
        "y": 8
      }
  ]
]

Thanks for the help!

[nlohmann]

Can you provide more information? What do you mean with concatenate?