The Julia C++ Interface

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

REPL Screenshot


You will need to install Julia v0.5-dev with some special options.

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 0.5 or greater), 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 """  

# 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" <

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 
       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(Dates.now())
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"

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 libArrayMaker.so

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 * "/libArrayMaker.so", 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()