How to add a new key:value pair to an existing json object

[peterritter]

Hi
Thanks for this library! I do find the 'user guide' rather sparse however. I can't figure out how to add a key:value pair to a json object that already exists. Say I have :
JSON j = JSON::object{
{ "key1", "value1" },
{ "key2", false },
{ "key3", JSON::array { 1, 2, 3 } },
{ "key4", 666.333 },
{ "key5", JSON::array { 1, 2, JSON::object{ { "key1", "value1" },
{ "key6", false } } } }
};

Now I want to add another key:value pair like :
j.add("key", "value"); //no add() member!

How would I do this?

furthermore, how do I change an existing value?

j["key5"] = "something else";

Any way of doing this?

Many thanks, Peter

[skabbes]

The Json interface is immutable (by design), however you can build up json objects by using Json::object (which is just a map<string, Json>underneath).

• Json is immutable.
• Json::array is actually vector<Json> and is therefore mutable
• Json::object is actually map<string, Json> and is therefore mutable

Json::object mutable_json = Json::object {
    { "key1", "value1" },
    { "key2", false },
    { "key3", JSON::array { 1, 2, 3 } },
    { "key4", 666.333 }
};
mutable_json["key5"] = string {"something else"};

Json immutable_json = Json {mutable_json};

This makes sense (to me) because there is no guarantee that a Json is an object - what would it mean to do this?

Json value = 5;
value["new_key"] = 6; // doesn't compile

[peterritter]

Hello skabbes

Many thanks for the response! The maintainer of the library may want to add your explanation to the user guide! I'm not really a JSON specialist, so I can't say what is 'legal' and what is not according to the json specification. However, from a purely practical perspective, I always think of the 'root' of the 'object' as a 'json object'. Would it really be 'Json' if it wasn't?

I am passing json objects around the network, as a quick way of putting messages together, and I needed to add an 'identifier token' to each message, and I just couldn't figure out how to do that. I did mange eventually to add an 'insert' function after struggling for quite a while.

Thanks

[maverick447]

hello skabbes,
Thanks for the tip! I am able to add a node but I need to add a Dictionary within a new entry into the map how would go ahead with it?

instead of this
mutable_json["key5"] = string {"something else"};
I need something like
mutable_json["key5"] = { add a name and value pair }

[skabbes]

@maverick447 give this a try:

mutable_json["key5"] = Json::object {
  { "new_key", "new_value" }
};

[maverick447]

HI skabbes,
Thanks!