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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.

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


dynamic_cast


reinterpret_cast


static_cast
```
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;


}
```

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);


  }

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.

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