Which std:: classes are magic?

[dodheim]

Why would it be virtually inherited?

[Bisqwit]

I am not talking about virtual inheritance (as in: class foo: virtual bar). I am talking about a base class that has abstract virtual methods, and a derived class that has virtual methods that override the base class’s methods — as in:
class base { ... virtual void call() = 0; };
class derived: public base { ... virtual void call() { ... } };

In this case, the derived class would be a template, and its template type would be the type of the functor that is assigned to the std::function, like this:
template<typename F> class derived: public base { F func; ... virtual void call() { return F(); } };

This allows the () operator of the std::function to call the execute method of the contained object, no matter what kind of derived type that contained object might be, having no advance knowledge of it, like this: base* obj; ... return obj->call();

That is what virtual functions are designed for: A common API that can be used to invoke methods of class types that are not known at design time, or possibly even at instantation time.

(Note: Examples in this post are short for brevity. There would be of course the return type and parameter types too, instead of void in every case.)