Method from_json_key

proto/fbe_json.h:378–386

struct KeyReader
{
    static bool from_json_key(const TJson& json, T& key)
    {
        std::string str;
        if (!FBE::JSON::from_json(json, str))
            return false;

        std::istringstream(str) >> key;
        return true;
    }
};

template <class TJson, typename T>