Is there any way to handle structures with const members?

[lukegv]

Hello!

I have some data structures that contain const members in order to maintain a level of thread safety within the application.
When writing from/to_json functions, I have run into a couple of problems that I am unable to navigate currently.
Here is what I have:

struct DictionarySuggestion
{
    const std::string dictionaryName;
    const std::vector<std::string> suggestions;
};

and

void from_json(nlohmann::json &json, DictionarySuggestion&suggestion)
{
    std::vector<std::string> *suggestionPtr = const_cast<std::vector<std::string> *>(&suggestion.suggestions);
    *suggestionPtr = json["suggestions"];
    std::string *dictionaryPtr = const_cast<std::string *>(&suggestion.dictionaryName);
    *dictionaryPtr = json["dictionaryName"];
}

void to_json(const nlohmann::json &json, const DictionarySuggestion&suggestion)
{
    json["dictionaryName"] = suggestion.dictionaryName;
    json["suggestions"] = suggestion.suggestions;
}

In the to_json method, I am getting an error when I try to assign dictionaryName:
C++ no operator matches these operands: operand types are: const nlohmann::json_abi_v3_11_2::basic_json<std::map, std::vector, std::string, bool, int64_t, uint64_t, double, std::allocator, nlohmann::json_abi_v3_11_2::adl_serializer, std::vector<uint8_t, std::allocator<uint8_t>>> = const std::string
And a similar error for the suggestion type also

How can I assign a const std::string to the nlohmann::json object?

Note: I am aware that const_cast can lead to undefined behaviour, but I am not sure if there is another way to accomplish what we are trying to do, so it may be a necessary evil for the time being. If anyone can suggest another approach, I'd be happy to hear it.

[gregmarr]

There is no safe way to write from_json for this. What is it that holds your DictionarySuggestion objects? You may need to do the handling at that level.