is it possible merge two json object

some function like this
json1.join(json2) or
json1.merge(json2) or
json1 += json2

This is an interesting question. Currently, there is no insert(InputIt first, InputIt last) function such as std::map::insert.

(I should definitely have another look at all the overloads of insert of the STL containers. For instance, std::map::iterator insert( const_iterator hint, const value_type& value ); exists, but an insert function with the same signature is currently only allowed for arrays.)

I have also needed this type of functionality. For the time being until this is closed, I have been using this code which modifies the json object in place.

One thing to consider is the desired behavior when j1 and j2 have matching keys. In this case j1 is over-written by values in j2.

Hopefully this helps for now.

void extendJson(json &j1, const json &j2) {
    for (const auto &j : json::iterator_wrapper(j2)) {
        j1[j.key()] = j.value();
    }
}

Thanks for the hint. I shall have a look at the insert functions of the STL containers and propose an extension here.

✨ proposal for #428

Here is a proposal for an insert function that takes an iterator range and can merge an object into another: 97a25de. Feedback very appreciated!

Well in python it interprets json data as an dict and then it could append (or insert) . In that case append is an alias of insert that would append to the end of the json, but insert would allow you to specify where to insert entries at. Maybe this library could copy some of that logic even though it would not be an dict.

I would prefer an shortcut that would overload operator>> where if it is another json object it would move the data over to the other one and return the object with it merged so when someone does this.

json2 >> json1;

That json2's data would move to json1 (or so it would seem like it).

@AraHaan

@AraHaan What exactly do you propose? We have several overloads for push_back and insert. And Python only interprets JSON objects as dict. Arrays are mapped to lists.

@AraHaan

@AraHaan I fear that overload of operator>> would be very unintuitive for C++ programmers.

Thanks，It's good for me！

Merged with 4b316ec.

Great this has been added; #252 addresses a similar issue and I have used

json merge( const json &a, const json &b )
{
    json result = a.flatten();
    json tmp = b.flatten();
    for ( auto it = tmp.begin(); it != tmp.end(); ++it )
        result[it.key()] = it.value();
    return result.unflatten();
}

I found the scenario below slightly counter-intuitive using the new approach,

json j1 = { {"a",1} };
json j2 = { {"a",2} };
j1.insert(j2.begin(), j2.end()); // --> a=1

But perhaps this is also the behaviour of maps?

std::map::insert does not overwrite either.

We use std::map::insert internally to provide the least surprising behavior. I also would have needed to look up the behavior in case of a duplicate.

That makes sense. I think the solution is simply to pay attention to order when merging,

std::map<int, int> goodKeys;
std::map<int, int> betterKeys;
betterKeys.insert(goodKeys.begin(), goodKeys.end());

Thanks!

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is it possible merge two json object #428

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nlohmann commented on Jan 15, 2017

nlohmann commented on Jan 15, 2017

andnp commented on Mar 23, 2017

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