SWIG - C++

Sun 2026/09/06 Languages SWIG / cpp / python / ruby / javascript

Overview

• copyright: Copyright and License
• repo status: Repo Information
• installation: Common Setup

Summary

A discussion and project for using SWIG C++ for Python, Ruby and javascript functions.

I had a need to call the same C++ functions from multiple scripting languages: python, ruby, javascript. This project is an example of how to set that up.

Other good sources for SWIG & python:

• Py Tutorial
• Swig Tutorial

Next Steps

For SWIG documentation, see Swig 4.3 doc

$ swig -version
SWIG Version 4.0.1
Compiled with g++ [x86_64-pc-linux-gnu]
Configured options: +pcre
Please see http://www.swig.org for reporting bugs and further information

• current calls only use simple functions. Need to try on various other data structures:
  • strings
  • arrays
  • structs
  • unions
  • JSON
  • enums (i.e. read-only)
  • pointers (is this needed?)

Common

• The source code is in src directory. The example.cpp and .h come from (with minor changes) SWIG example
• example.i is used across all sub-targets and is used by swig to configure what is made available to the language
  • I used #include "../src/example.h" and %include. The assumption is that all and only exported functions and variables are named in the example.h file. This may not be true for all projects or all scripting languages.
  • the functions in example.c are simple. No unions, no structs, no char* or kinds of pointers, no arrays, i.e. simple. Other C constructs may cause some additional definitions to be needed.

Do a clean rebuild and test

• zpm clean - wipes out all the output directories and the cmake build.
• zpm install full - insures latest ruby, python, node are installed
• zpm mk-gen - generates Makefile for the current OS
• zpm mk-build - builds using make
• zpm doit - runs scripts for all languages

Overall:

# expected stdout:
zpm doit
==== versions
python: Python 3.12.3
ruby  : ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [x86_64-linux]
node  : v24.16.0
==== run
           :     python        ruby       node 
fact       :        720         720        720 
my_mod     :          3           3          3 
before     :        3.0         3.0        3.0 
after      :        5.1         5.1        5.1 
sum1       :        261         261        261 
sum2       :        261         261        261 
append     :     abc-ok      abc-ok     abc-ok 
app before :        3.0         3.0        3.0 
app after  :        5.2         5.2        5.2 
app append :     xy-app      xy-app     xy-app 
==== convert to hex
python     :    01 02 03 FF 05 06 07 08   09 0A 0B 0C 0D 0E 0F 10 
ruby       :    01 02 03 FF 05 06 07 08   09 0A 0B 0C 0D 0E 0F 10 
node       :    01 02 03 FF 05 06 07 08   09 0A 0B 0C 0D 0E 0F 10 

• fact - factorial, passes in an integer, returns an integer
• my_mod - modulus, passes in two integers, returns an integer
• before - gets the default value of global variable my_variable
• after - sets my_variable and then gets it again
• sum1 - sums an array of integers
• sum2 - sums an array of bytes (uint8_t)
• append - concats "-ok" to given string
• app before - App::my_variable default value
• app after - App::my_variable is set then gets it again
• app append - App.append() call, concats "-app" to given string
• second section shows to_hex() output

Run test.py

Output directory: out/py

The import loads the factorial function, the mod function and cvar. "cvar" is used to access global C variables. Note these are read/write.

from out.py.example import fact, my_mod, cvar

Expected output:

$ python3.12 test.py
# see doit output above, column starting with "python"

Run test.rb

Output directory: out/rb

The require is very simple in ruby, it loads the .so:

require_relative './out/rb/example.so'

Expected output:

$ ruby test.rb
# see ./doit output above, column starting with "ruby"

Run test.js

For good examples of calls in node_wrap.h, see Node Addon For node_api.h doc & usage: Node n-api

Output directory: out/js

The require is very simple in javascript, it loads the .node:

const example = require("./out/js/example.node")

Expected output:

$ node test.js
# see ./doit output above, column starting with "javascript"