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    May 2000
    KY, USA

    C++ Casting: What are the C++ casting operators?

    Q: What are the C++ casting operators?

    A: Casting means you change the representation of a variable by changing its type to a different one. In order to type-cast a simple object to another you use the traditional type casting operator. For example, to cast a floating point number of type 'double' to an integer of type 'int':

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

    i = int (d);
    This is quite good for basic types that have standard defined conversions, however this operators can also been indiscriminately applied on classes and pointers to classes. ANSI-C++ standard has defined four new casting operators: 'reinterpret_cast', 'static_cast', 'dynamic_cast' and 'const_cast' in order to control these types of conversions between classes...

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    • 'reinterpret_cast'

      'reinterpret_cast' casts a pointer to any other type of pointer. It also allows casting from pointer to an integer type and vice versa.

      This operator can cast pointers between non-related classed. The operation results is a simple binary copy of the value from a 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 enough large 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);
      'reinterpret_cast' treats all pointers exactly as traditional type-casting operators do.

    • 'static_cast'

      'static_cast' allows to perform any casting that can be implicitly performed as well as also the inverse cast (even if this is not allowed implicitly).

      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 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);
      'static_cast', aside from manipulating pointers to classes, can also be used to perform conversions explicitly defined in classes, as well as to perform standard conversions between fundamental types:

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

      'dynamic_cast' is exclusively used with pointers and references to objects. It allows any type-casting that can be implicitly performed as well as the inverse one when used with polymorphic classes, however, unlike static_cast, dynamic_cast checks, in this last case, if the operation is valid. That is to say, it checks if the casting is going to return a valid complete object of the requested type.

      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'

      This type of casting manipulates the const attribute of the passed object, either to be set or removed:

      class C {};
      const C *a = new C;
      C *b = const_cast<C *>(a);
      Neither of the other three new cast operators can modify the constness of an object.


      • It is undefined behaviour if the pointer is used to write on an constant object (an object declared as 'const').

      • The 'const_cast' operator can also change the 'volatile' qualifier on a type.

    Last edited by Andreas Masur; November 13th, 2005 at 08:06 AM.

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