Generic value from JSON to function

[paulocoutinhox]

Hi,

I have a small problem here that im searching a lot to solve.

I have this function:

void test(const std::any &param)
{
    auto value = std::any_cast<bool>(param);
    std::cout << "Received Value: " << value << std::endl;
}

And i have a generic code the read a JSON, get a function name and the parameter value, that can be bool, i32, or custom class.

// main
int main()
{
    // list of mapping
    std::map<std::string, std::function<void(const std::any &)>> flist;
    flist["test"] = [] &MyClass::test;

    // method: test
    {
        // the json come from mobile (kotlin or swift)
        json j = json::parse(R"({
            "function": "test",
            "param": true
        })");

        auto function = j["function"].get<std::string>();
        auto param = j["param"].get<std::any>();
        flist[function](param);
    }
}

But nlohmann json don't know std::any and i can't use bool because only my function test that can receive something that it will cast to correct value type. And the function test can't receive a nlohmann object or string to be parsed after, because it need receive an independent value to be casted after be called.

The error is:

/Users/paulo/Developer/workspaces/cpp/cpp-native-mobile/cxx/src/main.cpp:94:33: error: no matching member function for call to 'get'
        auto param = j["param"].get<std::any>();
                     ~~~~~~~~~~~^~~~~~~~~~~~~
/Users/paulo/Developer/workspaces/cpp/cpp-native-mobile/build/cxx/_deps/json-src/include/nlohmann/json.hpp:1731:10: note: candidate template ignored: substitution failure [with ValueTypeCV = std::any, ValueType = std::any]: no matching member function for call to 'get_impl'
    auto get() const noexcept(
         ^
/Users/paulo/Developer/workspaces/cpp/cpp-native-mobile/build/cxx/_deps/json-src/include/nlohmann/json.hpp:1772:10: note: candidate template ignored: substitution failure [with PointerType = std::any]: no matching member function for call to 'get_ptr'
    auto get() noexcept -> decltype(std::declval<basic_json_t&>().template get_ptr<PointerType>())
         ^               

Can anyone help me?

[falbrechtskirchinger]

The usual recommendation would be to implement a conversion function for std::any. Unfortunately, std::any is currently explicitly excluded from being usable with get().
The only option is manually retrieving the value as stored or using a convertible type and converting the result to std::any.

[paulocoutinhox]

:(

This remove the dynamic part of the project. I need find a way to convert it dynamically. It something is wrong, a try/catch get it, dont have problem.

[paulocoutinhox]

I can also make something like this (pseudo code):

struct FunctionData
{
    std::function<void(const T &)> func;
    T type;
};

auto a = FunctionData{&MyClass::test, bool};
auto param = j["param"].get<a.type>();
a.func(param);

The code is wrong, but i want do something like this.

[falbrechtskirchinger]

How about something like this?

void test(bool value) {
    std::cout << "Received Value: " << value << std::endl;
}

int main() {
    std::map<std::string, std::function<void(const json &)>> flist;
    flist["test"] = [](const json &j) {
        bool value = j.get<bool>();
        test(value);
    };

    json j{{"function", "test"}, {"param", true}};
    auto function = j["function"].get<std::string>();
    flist[function](j["param"]);
}

[paulocoutinhox]

This is possible and work, but the problem is this line bool value = j.get<bool>();. The type bool and json need be dynamic or outside this, because it is a convertion layer that transform json into objects. I cant use json object on mapping.

Need be something like:

1. Mapping receive function pointer and the type.
2. When some data come, it get the function by name and get the item inside map.
3. Check what adapter is used.
4. If is json adapter, it try convert the json string that come to the type (this is the problematic part) from mapping.

[falbrechtskirchinger]

Looks like you need another mapping to convert the JSON value. I don't have enough information to figure this step out for you.