Item 44. Casts

Difficulty: 6

How well do you know C++'s casts? Using them well can greatly improve the reliability of your code.

The new-style casts in standard C++ offer more power and safety than the old-style (C-style) casts. How well do you know them? The rest of this problem uses the following classes and global variables:

class  A { public: virtual ~A(); /*...*/ }; 
A::~A() { }

class B : private virtual A  { /*...*/ };

class C : public  A          { /*...*/ };

class D : public B, public C { /*...*/ };

A a1; B b1; C c1; D d1;
const A  a2;
const A& ra1 = a1;
const A& ra2 = a2;
char c;

This Item presents four questions.

1. Which of the following new-style casts are not equivalent to a C-style cast?

   const_cast 
   dynamic_cast
   reinterpret_cast
   static_cast
   

2. For each of the following C-style casts, write the equivalent new-style cast. Which are incorrect if not written as a new-style cast?

   void f() 
   {
     A* pa; B* pb; C* pc;
     pa = (A*)&ra1;
     pa = (A*)&a2;
     pb = (B*)&c1;
     pc = (C*)&d1;
   }
   

3. Critique each of the following C++ casts for style and correctness.

   void g() 
   {
     unsigned char* puc = static_cast<unsigned char*>(&c);
     signed char* psc = static_cast<signed char*>(&c);
   
     void* pv = static_cast<void*>(&b1);
     B* pb1 = static_cast<B*>(pv);
   
     B* pb2 = static_cast<B*>(&b1);
   
     A* pa1 = const_cast<A*>(&ra1);
     A* pa2 = const_cast<A*>(&ra2);
   
     B* pb3 = dynamic_cast<B*>(&c1);
   
     A* pa3 = dynamic_cast<A*>(&b1);
   
     B* pb4 = static_cast<B*>(&d1);
   
     D* pd = static_cast<D*>(pb4);
   
     pa1 = dynamic_cast<A*>(pb2);
     pa1 = dynamic_cast<A*>(pb4);
   
     C* pc1 = dynamic_cast<C*>(pb4);
     C& rc1 = dynamic_cast<C&>(*pb2);
     }
   

4. Why is it typically unuseful to const_cast from non-const to const? Demonstrate a valid example in which it can be useful to const_cast from non-const to const.