A simple C++ interpreter written in JavaScript

JSCPP

This is a simple C++ interpreter written in JavaScript.

Try it out on github.io!

Purpose of the project

As far as I know, every public online C++ excuting environment requires backend servers to compile and run the produced executable. A portable and lightweight interpreter that can be run in browsers can be a fine substitute for those who do not intend to pay for such services.

I also want to make a strict interpreter. The reason being C++ has too many undefined and platform-dependent behaviors and popular C++ compilers tend to be an “over-caring mother” who tries to ignore or even justify the undocumented usages. The abuse of them should be avoided as much as possible IMO. For example, I do not want my students to take it as guaranteed that sizeof int produces 4, because on Arduino Uno, an int is a 2-byte value.

Currently, it is mainly for educational uses for a MOOC course I am running (and fun).

Prerequisites

• NodeJS version >= 0.11
• A modern browser

How to use

Installation

npm install JSCPP

or (to use lastest cutting-edge version or to contribute)

git clone https://github.com/felixhao28/JSCPP.git
cd JSCPP
npm install .

With NodeJS

var JSCPP = require("JSCPP");
var code =    "#include <iostream>"
            + "using namespace std;"
            + "int main() {"
            + "    int a;"
            + "    cin >> a;"
            + "    cout << a << endl;"
            + "    return 0;"
            + "}"
;
var input = "4321";
var exitcode = JSCPP.run(code, input);
console.info("program exited with code " + exitcode);

See demo/example.coffee for example.

Use debugger

As of 2.0.0, there is a simple but functional real debugger available.

A list of debugger API:

• methods
  • debugger.next(): one step further
  • debugger.continue(): continue until breakpoint
  • debugger.nextNode(): the AST node to be executed
    • sLine
    • sColumn
    • sOffset
    • eLine
    • eColumn
    • eOffset
  • debugger.nextLine()
  • debugger.type(typeName)
  • debugger.variable()
  • debugger.variable(variableName)
• properties
  • src: preprocessed source
  • prevNode: previous AST node
  • done
  • conditions
  • stopConditions
  • rt: the internal runtime instance
  • gen: the internal generator

var JSCPP = require("JSCPP")
var mydebugger = JSCPP.run(code, input);
// continue to the next interpreting operation
var done = mydebugger.next();
// if you have an active breakpoint condition, you can just continue
var done = mydebugger.continue();
// by default, debugger pauses at every new line, but you can change it
mydebugger.stopConditions = {
    isStatement: true
    positionChanged: true
    lineChanged: false
};
// so that debugger only stops at a statement of a new position
// or you can add your own condition, i.e. stops at line 10
mydebugger.conditions["line10"] = function (prevNode, nextNode) {
	if (nextNode.sLine=== 10) {
		// disable itself so that it only triggers once on line 10
		mydebugger.stopConditions["line10"] = false
		return true;
	} else {
		return false;
	}
};
// then enable it
mydebugger.stopConditions["line10"] = true
// we need to explicitly use "false" because exit code can be 0
if (done !== false) {
	console.log("program exited with code " + done);
}
// the AST node to be executed next
var s = mydebugger.nextNode();
// sometimes a breakpoint can be set without a statement to be executed next,
// i.e. entering a function call.
while ((s = mydebugger.nextNode()) == null) {
	mydebugger.next();
}
// the content of the statement to be executed next
var nextLine = mydebugger.nextLine();
// it is essentially same as
nextLine = mydebugger.src.slice(s.sOffset, s.eOffset).trim()

console.log("from " + s.sLine + ":" + s.sColumn + "(" + s.sOffset + ")");
console.log("to " + s.eLine + ":" + s.eColumn + "(" + s.eOffset + ")");
console.log("==> " + nextLine);
// examine the internal registry for a type
mydebugger.type("int");
// examine the value of variable "a"
mydebugger.variable("a");
// or list all local variables
mydebugger.variable();

A full interactive example is available in demo/debug.coffee. Use node -harmony demo/debug A+B -debug to debug “A+B” test.

With a modern browser

There should be a newest version of JSCPP.js in dist ready for you. If not, use npm run build to generate one.

Then you can add it to your html. The exported global name for this package is “JSCPP”.

<script src="JSCPP.es5.min.js"></script>
<script type="text/javascript">
	var code = 	"#include <iostream>"+
				"using namespace std;"+
				"int main() {"+
				"    int a;"+
				"    cin >> a;"+
				"    cout << a << endl;"+
				"    return 0;"+
				"}"
	;
	var input = "4321";
	var output = "";
	var config = {
		stdio: {
			write: function(s) {
				output += s;
			}
		}
	};
	var exitCode = JSCPP.run(code, input, config);
	alert(output + "\nprogram exited with code " + exitCode);
</script>

If you do not provide a customized write method for stdio configuration, console output will not be correctly shown. See demo/demo.html for example.

Run tests

npm run test

Q&A

Which features are implemented?

• (Most) operators
• Primitive types
• Variables
• Arrays
  • Multidimensional array with initializers.
• Pointers
• If…else control flow
• Switch…case control flow
  • Declarations inside switch block.
• For loop
• While loop
• Do…while loop
• Functions
• Variable scopes
• Preprocessor directives
  • Macro
  • Include

Which notable features are not implemented yet?

• Goto statements
• Object-oriented features
• Namespaces
• Multiple files support

How is the performance?

If you want to run C++ programs effciently, compile your C++ code to LLVM-bitcode and then use Emscripten.

Which libraries are supported?

See current progress in includes folder.

• iostream (only cin and cout and endl)
• cmath
• cctype
• cstring
• cstdio (partial)
• cstdlib (partial)

Bug report? Feedback?

Post it on Issues.

Changelog

• v1.0.0 (2015.3.31)
  • Formal release of this project.
• v1.0.1 (3.31)
  • This release is a mistake.
• v1.0.2 (3.31)
  • New examples.
  • Update README.
  • Renamed runtime and interpreter to start with upper case.
  • (dev-side) Grunt.
• v1.0.3 (4.1)
  • (dev-side) Fix dev-dependency on coffee-script.
  • (dev-side) Grunt watches.
  • (dev-side) Port to coffeescript
  • (dev-side) Refactoring
  • (dev-side) Reworked testing, now all tests are defined in test.json
  • Fixed a bug related to a.push(b).concat(c) in syntax parser (#1).
  • Added new tests
• v1.1.0 (4.2)
  • Fixed array initialization with 0 issue
  • Added support for reading string with cin
  • Member function should not be registered globally
  • Added new tests
  • Basic debugging support
• v1.1.1 (4.3)
  • Split debugger from example
  • (dev-side) Grunt only watches newer
  • Fix debug prev command
• v2.0.0 (4.11)
  • New
    • Real debugger!
  • Change
    • API: Now JSCPP.run is all you need
    • Runtime: The project uses es6, please use V8 with harmony flag
    • Deprecated: Removed old legacy profiling-replay debugging
    • Misc: Many other small changes
  • Dev
    • Major refactoring on interpreter using es6
• v2.0.1 (6.24)
  • Fix
    • Debugger variable scope issue
    • Readme example
    • An issue on Chrome Canary
    • Integer type promotion issue
    • Many small fixes
• v2.0.2 (7.31)
  • New
    • Wide char support (русский язык)
  • Fix
    • Truncating negative number to positive
    • Error message when overflow
    • Error when using function prototypes
  • Dev
    • test.json -> test.yaml
    • Transpile to es5 properly (dist/JSCPP.es5.js)
    • Minified version (dist/JSCPP.es5.min.js)
    • Online site uses minified es5 version now
    • A few dependencies have been updated
    • Added linux helper for running and debugging (bin/run & bin/debug)
• v2.0.3 (10.15)
  • New
    • Function pointer support
    • typedef support
  • Dev
    • Testing now uses test/test.coffee directly
    • Update PEG.js from 0.8.0 to 0.9.0
    • Please use NodeJS v4.0.0+
• v2.0.4 (12.10)
  • New
    • Function default arguments support
  • Fix
    • cin.get now works as expected
  • Dev
    • Update dev dependencies
• v2.0.5 (4.6)
  • New
    • ctime library
  • Fix
    • Supports functions returning pointer type
• v2.0.6 (9.29)
  • New
    • Implemented following methods from cstdlib, thanks to victorrseloy:
      • int getchar(void)
      • char *gets(char *str)
      • int putchar(int char)
      • int puts(const char *str)
      • int scanf(const char *format, ...)
      • int sscanf(const char *str, const char *format, ...)
  • Fix
    • Fixed printf function
  • Dev
    • Update dependencies to latest
• v2.0.7 (-)
  • New
    • Support some basic class usages (see test\class_basics.cpp).