How to get a sum of values in array in Google Native Client inside C++?

Question

In my HTML
Let's say I have 2 input fields with values 3 and 4:

<form onchange="reload()">
  <h2>Input:</h2>
  <input type="number" id="val1" name="val1" value="3">
  <input type="number" id="val2" name="val2" value="4">
  <br><br>
  <h2>Output</h2>
  <input type="text" id="out" name="out" value="untouched by C++"><br>
</form>

In my JavaScript
I get the two values and push them into an array like so:

Module = document.getElementById('module');
var msg = [];
msg.push(Number(document.getElementById('val1').value));
msg.push(Number(document.getElementById('val2').value));

Then I send it to my C++ file to process the message

Module.postMessage(msg);

In my C++ file [ Here is where I am stuck. ]
The code I have to handle the message is below

virtual void HandleMessage(const pp::Var& var_message) {
  std::string message = var_message.AsString();
  pp::Var var_reply = pp::Var(message);
  PostMessage(var_reply);
}

The issue is that it handles a string [actually it crashes if I my msg is of type of an array].

What I want it to expect and accept is an array or an object.
Basically, something like this:

virtual void HandleMessage(const pp::Var& var_message) {
  pp::Var var_reply =  var_message[0] + var_messgae[1]; // I expect this to be 3+4=7
  PostMessage(var_reply);
}

Can somebody help me figure out how to expect an array or an object from JavaScript inside my C++ so that I could calculate values together and send the result back to JavaScript?

Answer 1

I have resolved the issue I had. The best approach is to use an object and pass the values as a JSON object, so

in JavaScript

values = {
        "val1": Number(document.getElementById('val1').value),
        "val2": Number(document.getElementById('val2').value)
    };
msg = JSON.stringify(values);
Module.postMessage(msg);

Then handle the message and send the response back to JavaScript

in C++:

In the header you need to add picoJSON to handle JSON and sstream to work with isstringstream:

#include <sstream>
#include "picojson.h"
using namespace std;

then later in the code:

virtual void HandleMessage(const pp::Var& var_message) {

        picojson::value v;

        // pass the message that was sent from JavaScript as a string
        // var_message.AsString() will be in form "{\"val1\":4,\"val2\":4}");
        // and convert it to istringstream

        istringstream iss2((string)var_message.AsString());

        // parse iss2 and extract the values val1 and val2
        string err = picojson::parse(v, iss2);

        int val1 = (int)v.get("val1").get<double>();
        int val2 = (int)v.get("val2").get<double>();

        // finally send the message and you'll see the sum in the JavaScript
        PostMessage( val1 + val2 );

  }