what are plugins in C++? - plugins

Question

Can someone please explain what the following paragraph means in simpler and elaborate terms?
Class_loader is a ROS-independent package that allows one to dynamically load exported C++ classes during runtime from a runtime library (i.e. .so/.dll file) and create objects of those classes. What makes a class loaded through class_loader different from just linking against a runtime library and using classes from it is that your code does not require the definition of the class (i.e. the header file for the class) in your client code. Classes loaded in this fashion are also often called plugins.

Answer 1

I don't know how much you know about shared libraries, ros ecosystem C++ inheritance but here are some basic things to know
ROS comes with two packages
class_loader: loads shared libraries during runtime
pluginlib: ros specific class loader based on class_loader. This helps you to find and load the libraries within ros ecosystem.
Here is a basic setup
You have a library called MyLibrary compiled as "libMyLibrary.so"
In this library you have an abstract interface class MyBase and several implementations of it:
// file: MyBase.h
class MyBase{
public:
virtual void foo() = 0;
};
-------
// file: MyBaseMainImplementation.h
class MyBaseMainImplementation: public MyBase{
public:
void foo();
};
// file: MyBaseMainImplementation.cpp
void MyBaseMainImplementation::foo() { std::cout << "bar" << std::endl; };
// Make this class loadable from other libraries. This enables it to act like a
// plugin
CLASS_LOADER_REGISTER_CLASS(MyBaseMainImplementation, MyBase);
-------
// file: MyBaseOtherImplementation.h
class MyBaseOtherImplementation: public MyBase{
public:
void foo();
};
// file: MyBaseOtherImplementation.cpp
void MyBaseMainImplementation::foo() { std::cout << "foo bar" << std::endl; };
// Make this class loadable from other libraries. This enables it to act like a
// plugin
CLASS_LOADER_REGISTER_CLASS(MyBaseOtherImplementation, MyBase);
In a second library or executable (program) you want to use some implementation of MyBase but you don't know which is available during compile time. What you can do is use the class_loader to load any library during runtime which has the implementation of MyBase you want
// load libMyLibrary.so
class_loader::ClassLoader loader("libMyLibrary.so");
// create instance of a class from libMyLibrary.so
boost::shared_ptr<MyBase> my_base_impl =
loader.createInstance<MyBase>("MyBaseMainImplementation");
The only thing needed here is the header of MyBase class, i.e. MyBase.h
Back to the main question What are plugins in C++?
Well here the various implementation of MyBase, i.e. MyBaseMainImplementation and MyBaseOtherImplementation are plugins which can be loaded during runtime without linking against libMyLibrary.so. That's all there is to it.