CPP-PluginApp

This application is an example of how to create extensible applications using plugins in C++.

What the app does

The app calculates the area and perimeter of some figures. The necessary code for each figure is in a shared library in the plugins directory. The app tries to load every .so/.dll file in that folder at runtime.

Quick overview of the code

There are two important classes: plugin::Figure and plugin::FigureLoader. The first one is an abstract class that every plugin must be inherit from. The second one is a helper that uses system APIs to load libraries at runtime.

Class plugin::Figure

This class has one field for the name of the figure and three abstract methods:

• setData: It's used to initialize all required values to make the mathematics.
• area: Calculates and returns the value of the area.
• perimeter: Calculates and returns the value of the perimeter.

Also, for every derived class must be two methods, one to create a figure and one more to destroy it. These methods must be declared as "C" code. The code defines the macro DEFAULT_REGISTER_FIGURE for this purpose using default constructors of the class within a extern "C" block.

Class plugin::FigureLoader

Encapsulates the functionality to load a dynamic library and create a figure. It uses system APIs made in C. That's the reason why the functions need to be declared as C code. The constructor loads a shared library using dlopen() or LoadLibrary() function depending on the OS. After that, it is used the plugin::Figure *createFigure() function to allocate and initialize an object. When the object is destroyed the function void destroyFigure( plugin::Figure* ) is called and the figure and library are both released.

Compilation and execution

Using cmake

mkdir build     # Create a new directory
cd build        # Change to it
cmake ..        # Execute cmake 
make            # Execute make
./figures       # Execute the program

You can disable the build of plugins executing cmake as follows:

cmake -DBUILD_PLUGINS=OFF ..

Using only g++

g++ -o figures FigureLoader.cpp main.cpp -ldl   # Compile main program
cd plugins                                      # Change to plugins directory
g++ -o libcircle.so circle.cpp -fPIC -shared -I.. # Compile the library
cd ..                                           # Change to the parent
./figures                                       # Execute the program

Author

Ricardo Román. Email: reroman4@gmail.com. Telegram: @reroman