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C++ Dynamic Class Loading

Playing with a class that dynamically loads classes at runtime from shared objects.

DLClass allows you to load shared objects at runtime and create instances of the classes inside them.

A loadable class must be subclasses of a 'base' class that is known to the loading executable, and that base class must define create_t and destroy_t function pointer types, which DLClass uses to instantiate the loaded class. The shared object must contain implementations of the create and destroy functions, with the same type as create_t and destroy_t (DLClass assumes they have this type)

These functions are responsible for creating and destroying instances of the loaded class. DLClass returns a std::shared_ptr<T> with destroy given to it as the destructor function, so it's safe to let the shared_ptr go out of scope (it'll correctly destroy the object)

The instance of DLClass that created the object need not remain alive until the objects it created are destroyed. DLClass maintains a shared_ptr to a 'shared library' object responsible for closing the dynamic library. When a shared object handle is closed, the destroy function pointers become invalid. If a loaded object instance tries to destroy itself after the handle has been closed, it'll segfault when it tries to execute the destroy function. Each created object also maintains a shared_ptr to the 'shared library' object implicitly through lambda capture, so only once all created objects and the DLClass instance have been destroyed does the 'shared library' object get destroyed. The destructor of the 'shared library' object closes the handle.

I read about loading C++ classes from shared objects here

Files

DLClass does the lifting. It uses the POSIX dynamic linking API to load shared objects and get pointers to the necessary functions.

Polygon is the 'base class' that the executable knows about at compile-time.

Triangle and Square are classes that are made into the shared objects triangle.so and square.so and loaded by the main executable.

main.cpp is where you can see how to use DLClass.

One cool thing about DLClass is that, although make_obj is variadic, the number and type of arguments is checked at compile-time thanks to the required typedefs in the template argument. Try passing any number of ints or other types to DLClass<Polygon>.make_obj(). It'll only compile with exactly one argument of type int.