Over the past few weeks, one of the concepts that I’ve been experimenting with is plugin architecture.  The idea of having a core application which can be extended by shared objects without recompiling the core program.  Or, possibly, a way of defining more services in an application based around plugins.  What I’ve done so far hasn’t been much, but, I haven’t spent much time working on it.  When I started out on researching it, one of the things I wanted was to be able to was define a C++ object in a plugin and be able to instantiate that object in the main program.  So, this is what I have come up with.  There are three parts of this solution: the plugin class definition in plugin.hpp, the plugin in someplugin.cpp, and the loader code in loader.cpp.  I’ve also included a CMakeLists.txt file to compile it with cmake.

plugin.hpp

#ifndef _PLUGIN_HPP_
#define _PLUGIN_HPP_

#include <string>

namespace plugins
{
  class Plugin 
  {  
  public:
    virtual std::string toString() = 0;
  };
}

#endif

awesomeplugin.cpp

#include "plugin.hpp"

namespace plugins
{
  class AwesomePlugin : public Plugin
  {  
  public:
    // A function to do something, so we can demonstrate the plugin
    std::string toString()
    {
      return std::string("Coming from awesome plugin");
    }
  };
}

extern "C" 
{
  // Function to return an instance of a new AwesomePlugin object
  plugins::Plugin* construct()
  {
    return new plugins::AwesomePlugin();
  }
}

loader.cpp

#include <iostream>
#include <vector>
#include <dlfcn.h>
#include <boost/function.hpp>

#include "plugin.hpp"

typedef std::vector<std::string>             StringVector;
typedef boost::function<plugins::Plugin* ()> pluginConstructor;

int main (int argc, char** argv)
{
  // Assemble the names of plugins to load
  StringVector plugins;
  for(int i = 1; i < argc; i++)
  {
    plugins.push_back(argv[i]);
  }

  // Iterate through all the plugins and call construct and use an instance
  for(StringVector::iterator it = plugins.begin(); it != plugins.end(); it++)
  {
    // Alert that we are attempting to load a plugin
    std::cout << "Loading plugin \"" << *it << "\"" << it->c_str() << std::endl;

    // Load the plugin's .so file
    void *handle = NULL;
    if(!(handle = dlopen(it->c_str(), RTLD_LAZY)))
    {
      std::cerr << "Plugin: " << dlerror() << std::endl;
      continue;
    }
    dlerror();

    // Get the pluginConstructor function
    pluginConstructor construct = (plugins::Plugin* (*)(void)) dlsym(handle, "construct");
    char *error = NULL;
    if((error = dlerror()))
    {
      std::cerr << "Plugin: " << dlerror() << std::endl;
      dlclose(handle);
      continue;
    }

    // Construct a plugin
    plugins::Plugin *plugin = construct();
    std::cout << "[Plugin " << *it << "] " << plugin->toString() << std::endl;
    delete plugin;

    // Close the plugin
    dlclose(handle);
  }

  return 0;
}

CMakeLists.txt

# Project Stuff
cmake_minimum_required (VERSION 2.6)
project (PluginDemo)

# Default Options
add_definitions("-std=c++0x")

# Find Boost
find_package(Boost REQUIRED)
include_directories(${Boost_INCLUDE_DIRS})

# Pull in the project includes
include_directories(${PROJECT_SOURCE_DIR}/include)
set(LIBRARY_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/lib)
set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin)
set(LIBS ${LIBS} pthread boost_thread rt)

# Build the plugin experiment
add_executable(pluginloader src/loader.cpp)
target_link_libraries(pluginloader ${LIBS} dl)
add_library(awesomeplugin SHARED src/awesomeplugin.cpp)

Basically create a directory with the folders bin, lib, and src. Put loader.cpp, awesomeplugin.cpp, and plugin.hpp in src, and CMakeLists.txt in the directory. Open a terminal and run “cmake . && make”. Run the pluginloader program and pass it the path to the plugin’s .so in the lib folder. Here is the output from my computer.

nathaniel@XtremePC:~/Programming/Experimentation> cmake .
— The C compiler identification is GNU
— The CXX compiler identification is GNU
— Check for working C compiler: /usr/bin/gcc
— Check for working C compiler: /usr/bin/gcc — works
— Detecting C compiler ABI info
— Detecting C compiler ABI info – done
— Check for working CXX compiler: /usr/bin/c++
— Check for working CXX compiler: /usr/bin/c++ — works
— Detecting CXX compiler ABI info
— Detecting CXX compiler ABI info – done
— Boost version: 1.46.1
— Configuring done
— Generating done
— Build files have been written to: /home/nathaniel/Programming/Experimentation
nathaniel@XtremePC:~/Programming/Experimentation> make
Scanning dependencies of target awesomeplugin
[ 50%] Building CXX object CMakeFiles/awesomeplugin.dir/Plugins/awesomeplugin.cpp.o
Linking CXX shared library lib/libawesomeplugin.so
[ 50%] Built target awesomeplugin
Scanning dependencies of target pluginloader
[100%] Building CXX object CMakeFiles/pluginloader.dir/Plugins/loader.cpp.o
Linking CXX executable bin/pluginloader
[100%] Built target pluginloader
nathaniel@XtremePC:~/Programming/Experimentation> bin/pluginloader lib/libawesomeplugin.so
Loading plugin “lib/libawesomeplugin.so”
[Plugin lib/libawesomeplugin.so] Coming from awesome plugin
nathaniel@XtremePC:~/Programming/Experimentation>

– Teknoman117