Why in some cases can I call a member function without an object?

Question

I am working with a numerical library deal.ii, within which a lot of numerical tools are integrated. What I found weird is that I can call member functions directly without defining an object. For example, I can directly call

Vectortools::interpolate_boundary_condition();

Could you tell me when I can directly call the member functions without defining an object? Thank you!

Answer 1

There's two cases you can do this:

1. The member function is declared static -- in that case it is basically a free function but scoped to the class. Notably, this cannot be used in a static function since an object is not required to invoke it.

2. When you are within a member function whose this pointer is implicitly convertible to a pointer to the type being invoked on (Vectortools in this case). Note that this is invoked on an object (implicitly *this). This can be used to invoke an inherited member that is being overridden or hidden:

   class A {
   public:
     virtual void foo();
   };
   
   class B : public A {
   public:
     virtual void foo() override;
   };
   
   void B::foo() {
     // Do something
   
     // Invoke the method we've overidden from A.
     A::foo();
   
     // Then do something else
   }