c++,json,c++11 , I think assigning JsonValue values to JsonValues might be very slow, in comparison to having a Value be a key


I think assigning JsonValue values to JsonValues might be very slow, in comparison to having a Value be a key

Question:

Tag: c++,json,c++11

Json::Value is located in:

#include <json/value.h>

I have a result Json::Value object being created which will be pushed back to the user. The issue with it is that it takes a super long time to complete, which boggles my mind.

I think that there is something off with Json::Value, maybe when assigning a value, i should be storing an address to effectively make pointers. Right now, I see something like:

//C++11
Json::Value a;
Json::Value b;
Json::Value c;
for ( /* interator over array*/){
    Json::Value d;
    d = iterator;
    a.append(d);
}
b["features"] = a;  //this line takes 1.06 seconds to do.

for ( /*iterator for another array*/){
    Json::Value e;
    e = iterator;
    c.append(e);
}
b["class"] = c;     //this line takes 2.817 seconds.

I am trying to figure out the exact specifications, but im fairly certain that each of the tagged items are maybe doing a hard copy. I was thinking to make this faster, since the item is already created, pointers would be useful, but i wasnt sure of the specifics as to what is going on here. Why do these calls take so long? Is there an alternate way to create these json objects since they already exist, or does It NEED to take as long as it does?

My ultimate goal is to optimize this set of code since i feel those assignments of variables should be faster.


Answer:

Assuming that you are using JsonCpp.

What you would want to do is something like:

b["features"] = std::move(a);

This would move-assign the Value rather than copy-assigning it. Unfortunately, that class does not declare a move-assignment operator and one will not be implicitly generated.

Fortunately, that class defines a swap function which you could use to prevent copying.

b["features"].swap(a);

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