How to call different functions according to the template type parameter?

Question

I'm using nlohmann::json to parse json string. I implement a util function GetValue() to retrieve object fields.

template<typename T1, typename T2>
bool CheckType(T1& it, T2& val) { return false; }

template<typename T1>
bool CheckType(T1& it, bool& val) { return it->is_boolean(); }

template<typename T1>
bool CheckType(T1& it, std::string& val) { return it->is_string(); }

....

template<typename T>
bool GetValue(nlohmann::json obj, std::string key, T& value, std::string &err) {
    auto it = obj.find(key);
    if (it != obj.end()) {
        if (CheckType(it, val)) {
            value = it->get<T>();
            return true;
        } else {
            err = key + " type error";
        }
    } else {
        err = key + " not found";
    }
    return false;
}

The function CheckType() looks ugly. What's the elegant way to do this?

max66 · Accepted Answer · 2019-06-21 10:05:07Z

1

Not sure but, if the get() support a throwing in case of bad type, It seems to me simpler write something as

template<typename T>
bool GetValue(nlohmann::json obj, std::string key, T& value, std::string &err) {
    auto it = obj.find(key);
    if (it != obj.end()) {
       try {
          value = it->get<T>();
          return true;
       }
       catch (...) { // catching the correct exception; see library documentation
           err = key + " type error";
       }
    } else
        err = key + " not found";

    return false;
}

completely avoiding the CheckType() functions.

answered Jun 21, 2019 at 10:05

max66

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2 Comments

jasonxia Over a year ago

using exception is a good idea for this case:). but I still want to find a better way to call different functions according to different T.

max66 Over a year ago

@jasonxia - here the problem if the lack (I suppose) of a method of type isType<T>() (or, maybe better, isConvertibleTo<T>()) in the library

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