Add entries to existing Json::object?

[cdeil]

Is it possible to add entries to an existing Json::object and use it like a Python dictionary?

Json data = Json::object {};
data["this"] = 42; // doesn't work
// or maybe data.append or some other way to do it?

If it's not possible at the moment ... is it possible to add this feature?

[artwyman]

The Json type is immutable, but the Json::object type is just a std::map, so your code would work if the first line created a Json::object instead. You can use that map to build whatever data you want, then wrap it in as Json(data) when you're done modifying it. You can also extract the map from a Json using object_items(), copy it, mutate it, and use it to create a new Json, similar to a builder pattern.

[cdeil]

@artwyman Thanks for that info!

Your suggestion works nicely ... except if I want to fill a hierarchical Json::object incrementally I have to create temp local Json::object variables, right?

int main() {
    Json::object data;
    data["k1"] = 1;
    data["k2"] = "v";
    // this doesn't work:
    data["k3"] = Json::object();
    data["k3"]["k4"] = 42;
    // this works:
    Json::object k3;
    k3["k4"] = 42;
    data["k3"] = k3;
    return 0;
}

Or is there a way to have data["k3"] be non-const so that I can fill it?
Here's the error I get:

est.cpp:170:22: error: no viable overloaded '='
    data["k3"]["k4"] = 42;
    ~~~~~~~~~~~~~~~~ ^ ~~
./json11.hpp:63:7: note: candidate function (the implicit copy assignment operator) not viable: 'this' argument has type 'const json11::Json', but method is not marked const
class Json final {
      ^
./json11.hpp:63:7: note: candidate function (the implicit move assignment operator) not viable: 'this' argument has type 'const json11::Json', but method is not marked const
class Json final {
      ^
1 error generated.

[cdeil]

Should we close this issue or would it be useful if I added a short example how to do this to the README?

[artwyman]

Incremental construction won't work because the value type of the Json::object map is Json, which is immutable. You can build incrementally if you do it bottom-up. If you didn't care about the cost of copying, you could create a helper function which would take a Json, and a new key+value and return a new Json with that new value added.

Do you really need incremental updates here, though? For the case you mention, you could do it all in a single initializer like this:

Json data = {
    { "k1", 1 },
    { "k2", v },
    { "k3", Json::object { { "k4", 42 } }
};

[cdeil]

There's two reasons I wanted top-down incremental construction.

1. I already have Python code that does it this way and wanted to just edit the syntax line by line to make it work in C++.
2. The JSON objects I construct have 10 to 30 keys and I don't like C++ statements that span that many lines ... makes it hard to find the problem if a comma or curly brace is missing.

So now I'll code it as incremental bottom-up construction, which is OK.

@artwyman Thanks a lot for your help!!!