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

问题

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.

回答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.

来源：https://stackoverflow.com/questions/62378053/how-do-i-interface-with-extern-variables-from-cheerp-js

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