c++ passing json object by reference - c++

Question

In the below code, I am taking requests from a client, put them together on a json object on my server class and sending it to a pusher(directly connected to a website, putting my data in there so I can search data easily)
The code is working perfectly fine, but my manager said that I need to pass json by reference in this code, and I have no idea what to do.
On Server Class:
grpc::Status RouteGuideImpl::PubEvent(grpc::ServerContext *context,
const events::PubEventRequest *request,
events::PubEventResponse *response){
for(int i=0; i<request->event_size();i++){
nhollman::json object;
auto message = request->events(i);
object["uuid"]=message.uuid();
object["topic"]=message.type();
pusher.jsonCollector(obj);
}
...
}
On Pusher Class:
private:
nholmann::json queue = nlohmann::json::array();
public:
void Pusher::jsonCollector(nlohmann::json dump){
queue.push_back(dump);
}
void Pusher::curlPusher(){
std::string str = queue.dump();
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, str.data());
...
}
As much as I understand, I need to send the json object by reference. How can I do that?

Answer 1

The simple answer is to change
void Pusher::jsonCollector(nlohmann::json dump)
to
void Pusher::jsonCollector(const nlohmann::json& dump)
(note that if this is inside the class then Pusher:: is a non-standard visual studio extension).
This will reduce the number of times the object is copied from 2 to 1 however you can avoid the copy completely by using std::move:
void Pusher::jsonCollector(nlohmann::json dump){
queue.push_back(std::move(dump));
}
And call it with:
pusher.jsonCollector(std::move(obj));
If you want to enforce this behaviour to ensure that callers of jsonCollector always use std::move you can change jsonCollector to:
void Pusher::jsonCollector(nlohmann::json&& dump){
queue.push_back(std::move(dump));
}

Answer 2

Well, references are one of the many, many features, that distinguishes C from C++.
In other languages, like python or java, when you pass an object (not basic types) to a function and change it there, it is changed in the caller entity as well. In these languages, you don't have pointers, but you need to pass the object, not a copy.
That's what you have with references in C++. They are used like value types, but they are no copy.
Pointers can be nullptr (or NULL in C), references cannot. The address a pointer points to can be changed (assigned), you cannot change what object a reference refers to.
Have a look at this https://en.cppreference.com/w/cpp/language/reference for more information.