WebAssembly - Working with C++

Quiz

In this chapter, we are going to compile a simple C++ program to javascript and execute the same in the browser.

Example

C++ Program - Reversing a given number.

#include <iostream> 
int reversenumber(int n) { 
   int reverse=0, rem; 
   while(n!=0) { 
      rem=n%10; reverse=reverse*10+rem; n/=10; 
   } 
   return reverse; 
}

We have done the installation of emsdk in folder wa/. In same folder, create another folder cprog/ and save above code as reverse.cpp.

We have already installed emsdk in the previous chapter. Here, we are going to make use of emsdk to compile the above c code.

Compile test.c in your command prompt as shown below −

emcc reverse.cpp -s STANDALONE_WASM o reverse.wasm

emcc command takes care of compiling the code as well as give you the .wasm code.

Example − reversenumber.html

<!doctype html> 
<html>
   <head> 
      <meta charset="utf-8">
      <title>WebAssembly Reverse Number</title>
      <style>
         div { 
            font-size : 30px; text-align : center; color:orange; 
         } 
      </style>
   </head>
   <body>
      <div id="textcontent"></div>
      <script> 
         let reverse; 
         fetch("reverse.wasm")  
            .then(bytes => bytes.arrayBuffer()) 
            .then(mod => WebAssembly.compile(mod)) 
            .then(module => {return new WebAssembly.Instance(module) })
            .then(instance => { 
            
            console.log(instance); 
            reverse = instance.exports._Z13reversenumberi(1439898); 
            console.log("The reverse of 1439898 = " +reverse); 
            document.getElementById("textcontent")
               .innerHTML = "The reverse of 1439898 = " +reverse; 
         }); 
      </script>
   </body>
</html>

Output

The output is as follows −

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