Section 5.4 Advanced Class Type-casting

Until now, in order to type-cast a simple object to another we have used the traditional type casting operator. For example, to cast a floating point number of type double to an integer of type int we have used:

int i;
double d;
i = (int) d;

i = int (d);

// class type-casting
#include <iostream.h>

class CDummy {
    int i;
};

class CAddition {
	int x,y;
  public:
	CAddition (int a, int b) { x=a; y=b; }
	int result() { return x+y;}
};

int main () {
  CDummy d;
  CAddition * padd;
  padd = (CAddition*) &d;
  cout << padd->result();
  return 0;
}

Although the previous program in sintactically correct in C++ (in fact it will compile with no warnings on most compilers) it is code with not much sense since we use function result, that is a member of CAddition, without having declared an object of that class: padd is not an object, it is only a pointer which we have assigned the address of a non related object. When accessing its result member it will produce a run-time error or, at best, just an unexpected result.

In order to control these types of conversions between classes, ANSI-C++ standard has defined four new casting operators: reinterpret_cast, static_cast, dynamic_cast and const_cast. All of them have the same format when used:

reinterpret_cast <new_type> (expression)
    dynamic_cast <new_type> (expression)
     static_cast <new_type> (expression)
      const_cast <new_type> (expression)

(new_type) expression
new_type (expression)

reinterpret_cast

This operator can cast pointers between non-related classed. The operation results is a simple binary copy of the value from one pointer to the other. The content pointed does not pass any kind of check nor transformation between types.

In the case that the copy is performed from a pointer to an integer, the interpretation of its content is system dependent and therefore any implementation is non portable. A pointer casted to an integer large enough to fully contain it can be casted back to a valid pointer.

class A {};
class B {};
A * a = new A;
B * b = reinterpret_cast<B*>(a);

static_cast

Applied to pointers to classes, that is to say that it allows to cast a pointer of a derived class to its base class (this is a valid conversion that can be implicitly performed) and it can also perform the inverse: cast a base class to its derivated class.

In this last case the base class that is being casted is not checked to determine wether this is a complete class of the destination type or not.

class Base {};
class Derived: public Base {};
Base * a = new Base;
Derived * b = static_cast<Derived*>(a);

double d=3.14159265;
int i = static_cast<int>(d);

dynamic_cast

Checking is performed during run-time execution. If the pointer being casted is not a pointer to a valid complete object of the requested type, the value returned is a NULL pointer.

class Base { virtual dummy(){}; };
class Derived : public Base { };
 
Base* b1 = new Derived;
Base* b2 = new Base;
Derived* d1 = dynamic_cast<Derived*>(b1);   // succeeds
Derived* d2 = dynamic_cast<Derived*>(b2);   // fails: returns NULL

If the type-casting is performed to a reference type and this casting is not possible an exception of type bad_cast is thrown:

class Base { virtual dummy(){}; };
class Derived : public Base { };
 
Base* b1 = new Derived;
Base* b2 = new Base;
Derived d1 = dynamic_cast<Derived&*>(b1);   // succeeds
Derived d2 = dynamic_cast<Derived&*>(b2);   // fails: exception thrown

const_cast

class C {};
const C * a = new C;
C * b = const_cast<C*> (a);

typeid

typeid (expression)

// typeid, typeinfo
#include <iostream.h>
#include <typeinfo>

class CDummy { };

int main () {
  CDummy* a,b;
  if (typeid(a) != typeid(b))
  {
    cout << "a and b are of different types:\n";
    cout << "a is: " << typeid(a).name() << '\n';
    cout << "b is: " << typeid(b).name() << '\n';
  }
  return 0;
}

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