Why does static_cast(*this) to a base class create a temporary copy?

Question

I'm reading Effective C++ and came across this example:

class Window {                                // base class public:   virtual void onResize() { ... }             // base onResize impl   ... };  class SpecialWindow: public Window {          // derived class public:   virtual void onResize() {                   // derived onResize impl;     static_cast<Window>(*this).onResize();    // cast *this to Window,                                               // then call its onResize;                                               // this doesn't work!      ...                                       // do SpecialWindow-   }                                           // specific stuff    ...  }; 

The book says:

What you might not expect is that it does not invoke that function on the current object! Instead, the cast creates a new, temporary copy of the base class part of *this, then invokes onResize on the copy!

Why does static_cast (above code) create a new copy? Why not just just use the base class part of the object?

Answer 1

Because this code asks to create a new object. This code wants to make a Window object from *this — which can be done using the copy constructor of Window.

What you want instead is this:

static_cast<Window&>(*this).onResize();  //                ^ //                note the & 

This means I want to make a Window& from *this — which is an implicit conversion from a derived class' reference (*this is a SpecialWindow&) to a Window& reference.

However, it's better to just call the specific version of the member function onResize() you want to call:

Window::onResize(); // equivalent to this->Window::onResize(); 

Answer 2

That's because the code is casting to a value Window instead of a reference Window&. According to the standard, this form of casting is equivalent to calling (C++11 §5.2.9/4 = C++03 §5.2.9/2)

Window __t (*this); __t.onResize(); 

which invokes the copy-constructor of Window, and performs onResize on that copy.

(The proper way of calling a method of the superclass is

Window::onResize(); 

)