The Julia C++ Interface
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Latest commit 06cfb1f May 7, 2018


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The Julia C++ Foreign Function Interface (FFI) and REPL.

REPL Screenshot


You will need to install Julia v0.6 (or later).

Build requirements

In addition to the system requirements to build julia itself, the following are required:

  • Debian/Ubuntu: libedit-dev, libncurses5-dev
  • RedHat/CentOS: libedit-devel

Building Cxx

Launch julia (version 0.6 or later), and in the terminal type


How it works

The main interface provided by Cxx.jl is the @cxx macro. It supports two main usages:

  • Static function call @cxx mynamespace::func(args...)
  • Membercall (where m is a CppPtr, CppRef or CppValue) @cxx m->foo(args...)

To embed C++ functions in Julia, there are two main approaches:

# Using @cxx (e.g.):   
cxx""" void cppfunction(args){ . . .} """ => @cxx cppfunction(args)

# Using icxx (e.g.):
julia_function (args) icxx""" *code here*  """ end

The C++ REPL

This package contains an experimental C++ REPL feature. Using the package will automatically add a new pane to your REPL that is accessible by pressing the < key.

Using Cxx.jl:

Example 1: Embedding a simple C++ function in Julia

# include headers
julia> using Cxx
julia> cxx""" #include<iostream> """  

# Declare the function
julia> cxx"""  
         void mycppfunction() {   
            int z = 0;
            int y = 5;
            int x = 10;
            z = x*y + 2;
            std::cout << "The number is " << z << std::endl;
# Convert C++ to Julia function
julia> julia_function() = @cxx mycppfunction()
julia_function (generic function with 1 method)
# Run the function
julia> julia_function()
The number is 52

Example 2: Pass numeric arguments from Julia to C++

julia> jnum = 10
julia> cxx"""
           void printme(int x) {
              std::cout << x << std::endl;
julia> @cxx printme(jnum)

Example 3: Pass strings from Julia to C++

julia> cxx"""
         void printme(const char *name) {
            // const char* => std::string
            std::string sname = name;
            // print it out
            std::cout << sname << std::endl;

julia> @cxx printme(pointer("John"))

Example 4: Pass a Julia expression to C++

julia> cxx"""
          void testJuliaPrint() {
              $:(println("\nTo end this test, press any key")::Nothing);

julia> @cxx testJuliaPrint()
       To end this test, press any key

Example 5: Embedding C++ code inside a Julia function

function playing()
    for i = 1:5
            int tellme;
            std::cout<< "Please enter a number: " << std::endl;
            std::cin >> tellme;
            std::cout<< "\nYour number is "<< tellme << "\n" <<std::endl;

Example 6: Using C++ enums

julia> cxx"""
       class Klassy {
               enum Foo { Bar, Baz };
               static Foo exec(Foo x) { return x; }
# Access enum
julia> @cxx Klassy::Bar

# Pass enum as an argument
julia> @cxx Klassy::exec(@cxx(Klassy::Baz))

Example 7: C++ Hello World class

julia> using Cxx
julia> cxx"""#include <iostream>
       class Hello
               void hello_world(const char *now){
                   std::string snow = now;
                   std::cout << "Hello World! Now is " << snow << std::endl;
julia> hello_class = @cxxnew Hello()
julia> tstamp = string(
julia> @cxx hello_class -> hello_world(pointer(tstamp))
Hello World! Now is 2015-06-19T11:20:31

Example 8: Using C++ with shared libraries



class ArrayMaker
        int iNumber;
        float fNumber;
        float* fArr;
        ArrayMaker(int, float);
        float* fillArr();



#include "ArrayMaker.h"
#include <iostream>

using namespace std;

ArrayMaker::ArrayMaker(int iNum, float fNum) {
    cout << "Got arguments: " << iNum << ", and " << fNum << endl;
    iNumber = iNum;
    fNumber = fNum;
    fArr = new float[iNumber];

float* ArrayMaker::fillArr() {
    cout << "Filling the array" << endl;
    for (int i=0; i < iNumber; i++) {
        fArr[i] = fNumber;
        fNumber *= 2;
    return fArr;

Compiling into shared library

>> g++ -shared -fPIC ArrayMaker.cpp -o

Using in Julia

julia> using Cxx

# Importing shared library and header file
julia> const path_to_lib = pwd()
julia> addHeaderDir(path_to_lib, kind=C_System)
julia> Libdl.dlopen(path_to_lib * "/", Libdl.RTLD_GLOBAL)
Ptr{Void} @0x000000000728e6d0
julia> cxxinclude("ArrayMaker.h")

# Creating class object
julia> maker = @cxxnew ArrayMaker(5, 2.0)
Got arguments: 5, and 2
Cxx.CppPtr{Cxx.CppValue{Cxx.CppBaseType{:ArrayMaker},(false,false,false)},(false,false,false)}(Ptr{Void} @0x00000000060ab570)

julia> arr = @cxx maker->fillArr()
Filling the array

julia> pointer_to_array(arr, 5)
5-element Array{Float32,1}:


Replacement the current compiler instance

As of now Cxx.jl does not allow redefinition of a function. In order to allow defining the same function multiple times the current compiler instance can be replaced by typing

__current_compiler__ = Cxx.new_clang_instance()