Is it possible to emplace a key, value pair into a boost::json::value?

Is it possible to emplace or otherwise add a key/value pair into a boost::json::value, like you can into a boost::json::object?

boost::json::object obj;
 obj.emplace("timestamp", "2023-01-01T00:00:00");

Does there exist some sort of short notation, something like this?

boost::json::value val;
val["key"] = "value";

Or do you need to explicitly create a boost::json::object for such use?

As I understand it, the boost::json::value is a kind of a wrapper for various types of json data, does it work similarly to std::variant?

As I understand it, the boost::json::value is a kind of a wrapper for various types of json data, does it work similarly to std::variant?

Yes. See the interface here: https://www.boost.org/doc/libs/1_86_0/libs/json/doc/html/json/dom/value.html

The direct assignment could be:

val = {"key", "value"};      // ["key":"value"]
val = { {"key", "value"} };  // {"key":"value"}
val = {
    {"key", "value"},
    {"key2", 42},
    {"key3", true},
};                           // {"key":"value","key2":42,"key3":true}

To manipulate an existing object: as_object(), e.g.

val.as_object().emplace("more", val);

Demo program: Live On Coliru

#include <boost/json.hpp>
#include <iostream>
namespace json = boost::json;

int main()
{
    json::value val;
    val = {"key", "value"};   std::cout << val << std::endl;
    val = {{"key", "value"}}; std::cout << val << std::endl;
    val = {
        {"key", "value"},
        {"key2", 42},
        {"key3", true},
    };
    std::cout << val << std::endl;

    val.as_object().emplace("more", val);
    std::cout << val << std::endl;

    val.at("more").as_object().emplace("more", val);
    std::cout << val << std::endl;

    val.at("more").emplace_null();
    std::cout << val << std::endl;
}

Printing

["key","value"]
{"key":"value"}
{"key":"value","key2":42,"key3":true}
{"key":"value","key2":42,"key3":true,"more":{"key":"value","key2":42,"key3":true}}
{"key":"value","key2":42,"key3":true,"more":{"key":"value","key2":42,"key3":true,"more":{"key":"value","key2":42,"ke
y3":true,"more":{"key":"value","key2":42,"key3":true}}}}
{"key":"value","key2":42,"key3":true,"more":null}