Dynamic Class Loading for C++ on Linux

Recently, I have had two occasions to use this technique. In the first case, I was developing a moving object simulation. I wanted to provide users with the ability to add new types of moving objects without having access to the main source. In order to accomplish this, I defined a base class called entity, which provides the interface definition for any moving object in the simulation. A simplified version of the entity prototype is shown below.

class entity {
private:
   float xyz[3];  // position of the object
public:
   activate(float)=0; // tell the object to move
   render()=0;  // tell the object to draw itself
};

Thus, all entities have at least a position in three-space, and all entities can draw themselves. Most entities will have many other state variables besides position and may have many other member functions besides activate and render, but these are not accessible through the entity interface.

New entity types can be defined, incorporating whatever motion dynamics the user desires. At runtime, the program loads all the libraries in a subdirectory called Entity and makes them available to the simulation.

The second example comes from a recent project in which we wanted to create a library that could load and save images of various formats. We wanted the library to be extensible, so we created a base image_handler class for loading and saving images.

class image_handler{
public:
   virtual Image loadImage(char *)=0;
   virtual int saveImage(char *, Image &)=0;
};

The image_handler has two public functions, used to load and save images, respectively. The Image class is the library's basic object type for images. It provides access to the image data and some basic image-manipulation functions.

In this case, we were not interested in creating multiple objects of each type of image_handler. Instead, we wanted one instance of each image_handler that would handle loading and saving images of that type. Rather than registering a maker for each handler, we created a single instance of the handler in its library and registered a pointer to it with a global map. The map is no longer a factory, since it does not produce objects per se; it is more of a generic image loader/saver. The keys used here were strings representing file extensions (tif, jpg, etc.). Because a file format can have one of several different extensions (e.g., tiff, TIFF), each handler registers itself multiple times with the global map, once for each extension.

Using the library, a main program can load or save an image simply by invoking the correct handler via the file extension:

map <string, handler, less<string>> handler_map;
char *filename = "flower.tiff";
char ext[MAX_EXT_LEN];
howEverYouWantToParseTheExtensions(filename, ext);<\n>
// after parsing "flower.tiff" ext = "tiff"<\n>
Image img1 = handler_map[ext]->loadImage(filename);
// process data here
handler_map[ext]->saveImage(filename, img1);

Dynamic class loading allows us to create more extensible and more robust code. By using dynamic class loading combined with well-thought-out base class designs, we provide users with a practical means of extending our code.

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