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Variadic Remote Procedure Calls

Project description

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  • Need to bridge programming languages?

  • Tired of writing bindings?

  • Want to have Node.js or Python classes and objects that call corresponding C++ code under the hood, natively or remotely?

  • Details like memory management, exception handling, overload resolution should just work?

If your answer is yes, then read further.

Design Goals

Integration Speed vrpc’s most important goal is to provide C/C++ code to other languages (currently Node.js and Python) without having to write any bindings or having to touch any of the existing code.

Completeness Any C++ function with any signature, including callbacks and custom data-types can be bound. Return values and exceptions are properly forwarded. Memory allocation is transparently mapped to the remote-object’s lifetime.

Asynchronicity vrpc is completely asynchronous which is especially important if used remotely. Node.js’ async/await can be used to have synchronous API feeling running a highly-performing non-blocking code under the hood.

Ease of Use vrpc is a header only library having no dependencies besides the amazing JSON for Modern C++ library of Niels Lohmann which is shipped with vrpc and is already integrated. Building addons follows Node.js’ regular workflow using the node-gyp tool.


Say you had some existing C++ code looking like this:


class Foo {

  int _value;


  Foo(int value) : _value(value) {}

  int getValue() {
    return _value;


Then all you need to do is listing the functions you want to bind in a file:


#include "Foo.hpp"

// Register the constructor, provide class and arguments
VRPC_CTOR(Foo, int);

// Register member function, provide class, return type, function name and arguments
VRPC_MEMBER_FUNCTION(Foo, int, getValue);

During project installation, vrpc will compile the existing C++ code together with your binding file into a native language addon. Afterwards, in Node.js you can call the C++ code like:


'use strict'

const VrpcLocal = require('vrpc')
const addon = require('build/Release/vrpc_foo')

const vrpc = VrpcLocal(addon)

const foo = vrpc.create('Foo', 42)
console.log(foo.getValue()) // prints 42

or in Python like so:

from vrpc import VrpcLocal
import vrpc_foo  # imports the addon

vrpc = VrpcLocal(vrpc_foo)

foo = vrpc.create('Foo', 42)
print(foo.getValue()) # prints 42

NOTE: In both cases (Node.js and Python) foo really only is a proxy to the C++ object that was created. vrpc automatically injected all functions specified in the bindings file into that proxy, providing a native “Call & Feel” to the underlying C++ code.

Real Life Examples

C++ to Node.js

The npm project vrpc-nodejs-example is an example of another node-js project using vrpc as dependency (github is here).

C++ to Python

The github project vrpc-python-example binds the same example code as in the Node.js example, but makes it available to Python 3.

Setup and Compilation - Node.js

Install vrpc

In your project run:

npm install vrpc

Add a binding.gyp file

Add the file (see below for a template) to the root of your project. Modify it as needed to e.g. express additional include directories or dependent libraries.


  'variables': {
    'vrpc_path': '<!(if [ -e ../vrpc ]; then echo ../vrpc/vrpc; else echo node_modules/vrpc/vrpc; fi)'
  'targets': [
      'target_name': 'vrpc_foo',  # Name of the extension
      'defines': [],  # Any pre-processor defines you need
      'cflags_cc!': ['-std=gnu++0x', '-fno-rtti', '-fno-exceptions'],
      'cflags_cc': ['-std=c++14', '-fPIC'],
      'include_dirs': [  # Include dirs to be found
        # '<path/to/binding-file>' # Make sure your binding.cpp file is found
        # '<other/include/dir>'
      'sources': [  # Sources to be compiled
          # <your/src/to_be_compiled.cpp>
      'link_settings': {
        'libraries': [  # System library dependencies, e.g.
          # '-lpthread'
        'ldflags': [  # Use e.g. for extern lib in a non-standard location:
          # '-Wl,-rpath,\$$ORIGIN<runtime/path/to/extern/lib>',
          # '-L<!(pwd)</compiletime/path/to/extern/lib>'

NOTE: Mention include directories and source files using a relative path with respect to your project’s root.

Build your project

After running

npm install

you will find the native addon: build/Release/<target_name>.node.

HINT: You can also use node-gyp rebuild to (re-)build the addon and add the flag --verbose to see details of build step.

Setup and Compilation - Python 3

Install vrpc

pip install vrpc

Prepare your project’s

You have to modify your project’s to define the native extension:

from distutils.sysconfig import get_python_lib
from setuptools import setup, Extension, find_packages
import os

vrpc_path = os.path.join(get_python_lib(), 'vrpc')
vrpc_module_cpp = os.path.join(vrpc_path, 'module.cpp')

module = Extension(
    'vrpc_foo_ext',  # Name of the extension
        ('VRPC_MODULE_NAME', '"vrpc_foo_ext"'),  # Module name
        ('VRPC_MODULE_FUNC', 'PyInit_vrpc_foo_ext')  # Init function name
    include_dirs=[  # Include dirs to be found
      # '<path/to/binding-file>'  # Make sure your binding.cpp file is found
    sources=[  # Sources to be compiled
        # <your/src/to_be_compiled.cpp>
    extra_compile_args=['-std=c++14', '-fPIC'],

    # [...]  Whatever needs to be set up for your package
    install_requires=[  # Mention vrpc as dependency
    ext_modules=[module]  # Add the extension module as defined above

NOTE: As you can see from the Extension, it is important that the path to the prior installed vrpc dependency is found. Depending on your pip installation the generic solution above may not always work and may need manual tweaking.

Build your package

While you are developing run e.g.:

pip install -e .

Binding File Details

The binding file - typically named binding.cpp - is the only piece of code you really have to provide in order to make language bindings happen.

In the binding file you have to mention:

  1. All classes and functions you want to bind

  2. All custom C++ data-types you want to expose

Basically, vrpc uses only four different macros types to express all bindings.

1. Constructors

VRPC_CTOR(<className>, <args>)

Use this macro to register constructors with arguments. Repeat this macro for all overloads you need.

For constructors without arguments use:


2. Member functions

VRPC_MEMBER_FUNCTION(<className>, <returnValue>, <functionName>[, <args>])

Use this macro to register class member functions. Repeat this macro for all overloads you need.

For member functions with void return value use:

VRPC_VOID_MEMBER_FUNCTION(<className>, <functionName>[, <args>])

For constant member functions use:

VRPC_MEMBER_FUNCTION_CONST(<className>, <functionName>[, <args>])

3. Static functions

VRPC_STATIC_FUNCTION(<className>, <returnValue>, <functionName>, <args>)

Use this macro to register static functions. Repeat this macro for all overloads you need.

For static functions with void return value use:

VRPC_VOID_STATIC_FUNCTION(<className>, <functionName>[, <args>])

4. Callbacks


Use this macro if an argument of a function you bind reflects a callback. The provided arguments must match the expected signature of the callback.

Binding of custom data types

This feature is brought in by the json library ( which is shipped with vrpc (see documentation there for full details).

Say your existing code had a struct:

namespace ns {
  // a simple struct to model a person
  struct Person {
    std::string name;
    std::string address;
    int age;

Then on the top of your binding file (before the macros) add:

#include <json.hpp>

namespace ns {
  void to_json(json& j, const Person& p) {
    j = json{{<s>"name"</s>,}, {<s>"address"</s>, p.address}, {<s>"age"</s>, p.age}};

  void from_json(const json& j, Person& p) { =<s>"name"</s>).get<std::string>();
    p.address =<s>"address"</s>).get<std::string>();
    p.age =<s>"age"</s>).get<int>();

NOTE: Once you exposed you custom data-types you are ready to use them as arguments in the binding macros (see above). They automatically also work within all STL containers and even as arguments of callback functions!

More Elaborate Example

Say your existing C++ code looks like that:


#include <chrono>
#include <functional>
#include <thread>
#include <unordered_map>
#include <vector>
#include <stdlib.h>

namespace vrpc_example {

  struct Bottle {
    std::string brand;
    std::string country;
    int age;

  class Bar {


    typedef std::function<void (const std::string& /*type*/)> Callback;
    typedef std::vector<Bottle> Bottles;
    typedef std::unordered_map<std::string, Bottles> Assortment;

    Bar() = default;

    explicit Bar(const Assortment& assortment): _assortment(assortment) {

    static std::string philosophy() {
      return <s>"I have mixed drinks about feelings."</s>;

    bool hasDrink(const std::string& type) const {
      return _assortment.find(type) != _assortment.end();

    void addBottle(const std::string& type, const Bottle& bottle) {

    Bottle removeBottle(const std::string& type) {
      if (!hasDrink(type)) {
        throw std::runtime_error(<s>"Can't remove bottle of unavailable category"</s>);
      Bottles& bottles = _assortment[type];
      Bottle bottle =  bottles.back();
      if (bottles.size() == 0) {
      return bottle;

    void onEmptyDrink(const Callback& callback) {
      _callback = callback;

    void prepareDrink(const std::function<void (int)>& done) const {
      const int seconds = rand() % 4;

    Assortment getAssortment() const {
      return _assortment;


    Callback _callback;
    Assortment _assortment;


Then you can fully bind it like that:


#include <json.hpp> // needed to register custom data types
#include "Bar.hpp"

// NOTE: Do not include <vrpc.hpp>, even if you IDE complains

namespace vrpc_example {

  // Register custom type: Bottle
  void to_json(vrpc::json& j, const Bottle& b) {
    j = vrpc::json{{<s>"brand"</s>, b.brand}, {<s>"country"</s>,}, {<s>"age"</s>, b.age}};
  void from_json(const vrpc::json& j, Bottle& b) {
    b.brand =<s>"brand"</s>).get<std::string>(); =<s>"country"</s>).get<std::string>();
    b.age =<s>"age"</s>).get<int>();

  // Register constructors
  VRPC_CTOR(Bar, const Bar::Assortment&)

  // Register static functions
  VRPC_STATIC_FUNCTION(Bar, std::string, philosophy)

  // Register member functions
  VRPC_MEMBER_FUNCTION_CONST(Bar, bool, hasDrink, const std::string&)
  VRPC_VOID_MEMBER_FUNCTION(Bar, addBottle, const std::string&, const Bottle&)
  VRPC_MEMBER_FUNCTION(Bar, Bottle, removeBottle, const std::string&)
  VRPC_VOID_MEMBER_FUNCTION(Bar, onEmptyDrink, VRPC_CALLBACK(const std::string&))
  VRPC_MEMBER_FUNCTION_CONST(Bar, Bar::Assortment, getAssortment)

Once compiled into an addon you can run this pure Node.js code:


'use strict'

const EventEmitter = require('events')
const VrpcLocal = require('../vrpc/VrpcLocal')
const addon = require('../build/Release/vrpc_example')

// Create an event emitter
const emitter = new EventEmitter()

emitter.on('empty', what => {
  console.log(` - Oh no! The ${what} is empty!`)

// Create an instance of a local (native-addon) vrpc factory
const vrpc = VrpcLocal(addon)

console.log('Why an example at the Bar?')
console.log(' - Because', vrpc.callStatic('Bar', 'philosophy'))

// Create a Bar instance (using default constructor)
const bar = vrpc.create('Bar')

console.log('Do you have rum?')
console.log(bar.hasDrink('rum') ? ' - Yes' : ' - No')

console.log('Well, then let\'s get a bottle out of the cellar.')
bar.addBottle('rum', { brand: 'Don Papa', country: 'Philippines', age: 7 })

console.log('Now, can I have a drink?')
console.log(bar.hasDrink('rum') ? ' - Yes' : ' - No')

console.log('I would go for a "Dark and Stormy", please.')
bar.prepareDrink(seconds => {
  console.log(` - Here's your drink, took only ${seconds}s`)

console.log('Nice! I take another one. Please tell me, once the rum is empty.')
bar.onEmptyDrink({ emitter: emitter, event: 'empty' })
bar.prepareDrink(seconds => {
  console.log(` - Here's your drink, took ${seconds}s this time.`)

// Create another bar - already equipped - using second constructor
const neighborsBar = vrpc.create(
    rum: [
      { brand: 'Botucal', country: 'Venezuela', age: 8 },
      { brand: 'Plantation XO', country: 'Barbados', age: 20 }
    brandy: [
      { brand: 'Lustau Solera', country: 'Spain', age: 15 }
console.log('How is your neighbor sorted?')
console.log(' - Very well:\n', neighborsBar.getAssortment())

or if you prefer Python you can write:

from vrpc import VrpcLocal
import vrpc_example_ext  # Imports the extension

def _onEvent(event, *args):
    if event == 'empty':
        print(" - Oh no! The {} is empty!".format(args[0]))

def main():
    # Create an instance of a local (native-extension) vrpc factory
    vrpc = VrpcLocal(vrpc_example_ext)
    print("Why an example at the Bar?")
    print(" - Because {}".format(vrpc.call_static('Bar', 'philosophy')))

    # Create a Bar instance (using default constructor)
    bar = vrpc.create('Bar')

    print("Do you have rum")
    print(" - Yes" if bar.hasDrink('rum') else " - No")

    print("Well, then let's get a bottle out of the cellar.")
        {'brand': 'Don Papa', 'country': 'Philippines', 'age': 7}

    print("Now, can I have a drink?")
    print(" - Yes" if bar.hasDrink('rum') else " - No")

    print("I would go for a \"Dark and Stormy\", please")
    msg = " - Here's your drink, took only {}s"
    bar.prepareDrink(lambda seconds: print(msg.format(seconds)))

    print("Nice! I take another one. Please tell me, once the rum is empty.")
    bar.onEmptyDrink((_onEvent, 'empty'))
    bar.prepareDrink(lambda seconds: print(msg.format(seconds) + " this time"))

    # Create another bar - already equipped - using second constructor
    neighborsBar = vrpc.create(
            'rum': [
                {'brand': 'Botucal', 'country': 'Venezula', 'age': 8},
                {'brand': 'Plantation XO', 'country': 'Barbados', 'age': 20}
            'brandy': [
                {'brand': 'Lustau Solera', 'country': 'Spain', 'age': 15}
    print("How is your neighbor sorted?")
    print(" - Very well:\n{}".format(neighborsBar.getAssortment()))

if __name__ == '__main__':

The output will look something like this:


Why an example at the Bar?
 - Because I have mixed drinks about feelings.
Do you have rum?
 - No
Well, then let's get a bottle out of the cellar.
Now, can I have a drink?
 - Yes
I would go for a "Dark and Stormy", please.
 - Here's your drink, took only 3s
Nice! I take another one. Please tell me, once the rum is empty.
 - Here's your drink, took 2s this time.
 - Oh no! The rum is empty!
How is your neighbor sorted?
 - Very well:
 { brandy: [ { age: 15, brand: 'Lustau Solera', country: 'Spain' } ],
   [ { age: 8, brand: 'Botucal', country: 'Venezuela' },
     { age: 20, brand: 'Plantation XO', country: 'Barbados' } ] }

Hopefully, this example is more or less self-explanatory. It is shipped within this repository (see examples directory) and can be build and run using

BUILD_EXAMPLE=1 npm install
node examples/Bar.js

for Node.js and in Python like so:

BUILD_EXAMPLE=1 pip install -e . --user
python examples/

HINT: This example is also available as stand-alone Node.js or Python project, see above

The thing with the callbacks

There are two very different categories of callbacks:

  • Those you provide as function argument and are called exactly once. All kinds of done callbacks, indicating the completion of an asynchronous activity belong to this category.

  • Those which you register once, and which are called any number of times until you explicitly de-register them. All kinds of event callbacks that work in a publish/subscribe fashion fall into that category.

The example demonstrates this two different callbacks, prepareDrink belonging to the first and onEmptyDrink to the second category, respectively.

vrpc can handle both of them in their natural way, i.e. use callback functions that can be wrapped up to Promises and play nice with async/await patterns for category one. The event-like callbacks can be taken up by Node.js’ inbuilt EventEmitter and assigned an arbitrary event-name. In both cases, all callback arguments are perfectly forwarded.

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