binding c++ class I'm not able to find the constructor

Question

I'm working on binding a simple c++ class into javascript. I'm doing this via Emscripten 2.0.17.

I bound hello_world.cpp

#include <iostream>

class hello {
    
public:
    
    hello() { std::cout << "Hello world!!!\n"; }
    
};

#include <emscripten/bind.h>

namespace emcc = emscripten;

EMSCRIPTEN_BINDINGS(jshello) {
    emcc::class_<hello>("hello")
        .constructor<>();   
}

using this command from terminal ./emcc_path/emcc --bind hello_world.cpp -o world.html

Then I created a main.js file like the following

var hw = require("/home/kubuntu/Desktop/c++/hello_world/hello_world.js");
var x = new hw.hello();    
x.delete();

I tried to run with node 15.14.0 using node main.js I got the following error

/home/kubuntu/Desktop/c++/hello_world/hello_world.js:117
      throw ex;
      ^

TypeError: hw.hello is not a constructor
    at Object.<anonymous> (/home/kubuntu/Desktop/c++/hello_world/main.js:3:9)
    at Module._compile (node:internal/modules/cjs/loader:1092:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1121:10)
    at Module.load (node:internal/modules/cjs/loader:972:32)
    at Function.Module._load (node:internal/modules/cjs/loader:813:14)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:76:12)
    at node:internal/main/run_main_module:17:47

Does anyone can solve say me what's worng? thanks in advance

Answer 1

Exposing a class with Emscripten

If you want to expose a C++ class with Emscripten you need these steps, write your C++ code:

// hello_world.cpp
#include <iostream>

class Hello {
    
public:
    
    Hello() { 
        std::cout << "Hello world!!!\n" << std::endl;
        }

    void saySomething() {
        std::cout << "something" << std::endl;
    }
    
};

#include <emscripten/bind.h>

using namespace emscripten;

EMSCRIPTEN_BINDINGS(jshello) {
    class_<Hello>("Hello")
        .constructor<>()
        .function("saySomething", &Hello::saySomething);
}

Building with Emscripten

build the C++ code with Emscripten, assumed that you have run the emsdk_env.sh you need to run:

emcc --bind hello_world.cpp -o hello_world.js

Note that the output is -o hello_world.js.

Module in example code

Now with the new wasm ad js files, write your main.js to load the Module:

// main.js

var Module = require("./hello_world.js");

Module.onRuntimeInitialized = async function(){
    // uncommentthe code below to output the Module in the console
    // console.log('Module loaded: ', Module);
    var instance = new Module.Hello(); // this will print "Hello world!!!""
    console.log(instance); // this will pint the class as an object "Hello{}""
    instance.saySomething(); // this will print "something"
}

we wrap the code into a onRuntimeInitizialized function, in this way we are sure that our code is ready.

Test the code!

At this point you can test the code:

node main.js

The output in the console will be:

node main.js
Hello world!!!

Hello {}
something

Example code

You can test the code in this repository https://github.com/kalwalt/Emscripten-sketches/tree/main/hello_world_class

Happy coding with Emscripten!