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Here's an example of how you can use codecvt_byname to convert from Latin1 to UTF-8 and apply it for JSON manipulation:

#include <codecvt>
#include <fstream>
#include <iostream>
#include <locale>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main() {
  // Open input file in Latin1 encoding
  std::ifstream input("input.json");
  input.imbue(std::locale(input.getloc(), new std::codecvt_byname<char, char, std::mbstate_t>("ISO-8859-1")));

  // Read file contents into string
  std::string contents(std::istreambuf_iterator<char>{input}, {});

  // Convert from Latin1 to UTF-8
  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
  std::wstring wide_str = converter.from_bytes(contents);

  // Parse JSON string
  json data = json::parse(wide_str);

  // Access data in JSON object
  std::cout << data["name"].get<std::string>() << std::endl;

  return 0;
}

In this example, we open the input file using ifstream, and then imbue it with a locale that has a codecvt_byname facet. We specify the encoding as "ISO-8859-1" to indicate that the input file is in Latin1 encoding.

Next, we read the contents of the file into a string. Since the input file is in Latin1 encoding, the contents of the string will also be in Latin1 encoding.

We then create a wstring_convert object with a codecvt_utf8_utf16 facet to convert the string from Latin1 to UTF-8 encoding. We call the from_bytes function to perform the conversion, which returns a wide string (std::wstring).

Finally, we parse the wide string as a JSON object using the json::parse function, and then access the data in the JSON object as needed.

Note that the codecvt_byname facet may not be available on all platforms, and may have different names depending on the specific encoding you want to use. You can check the available codecvt facets on your platform by calling std::locale::facet with the appropriate type parameter.