How do I interface with extern variables from Cheerp/js?

Question

Cheerp is a C++ to js/wasm transpiler. Screeps is a programming videogame.

How do I read in the Game.time variable from my transpiled C++ code? (in screeps)

---

#include <cheerp/client.h>
#include <iostream>
using namespace std;

namespace client {
    class Game : public Object {
    public:
        static volatile double time;
    };  

    extern volatile Game &Game;
}

void webMain() {
    cout << __TIME__ << ": The current time is: " << client::Game.time << endl;
}

I have tried any number of variations on:

• extern, volatile and static
• references and pointers
• both client and cheerp namespaces
• Inheriting from Node/Object
• int32_t vs double vs float as a type

I seem to get either:

• NaN
• 0
• 1
• fatal type handling errors in the compiled code

How do I correctly interface with Javascript Objects and variables from within my C++ code? The cheerp documentation is VERY sparse to say the least...

---

Note: cheerp is never actually generating the proper Javascript. There's always some inconsistency as to how the Game object is handled and in a lot of scenarios it's incorrectly trying to index Game.d as an array instead of Game.time.

Answer 1

Classes declared in the client namespace should have no member fields.

To access external JS objects properties you need to add methods starting with get_ and set_, to respectively read and write to the property:

#include <cheerp/client.h>
#include <iostream>
using namespace std;

namespace client {
    class Game : public Object {
    public:
        double get_time();
    };  

    extern Game &Game;
}

void webMain() {
    cout << __TIME__ << ": The current time is: " << client::Game.get_time() << endl;
}

Also, you don't need to use volatile here.