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Language: C++
Expertise: Intermediate

Mar 3, 2003

The const_cast operator takes the form:

 
    const_cast<T> (expr)

It is used to add or remove the ``const-ness'' or ``volatile-ness'' from a type. Consider a function, f, which takes a non-const argument:

 
   double f( double& d );

Say you wish to call f from another function g:

 
   void g (const double& d)
   {
     val = f(d);
   }

Because d is const and should not be modified, the compiler will complain because f may potentially modify its value. To get around this dilemma, use a const_cast:

  
  void g (const double& d)
   {
      val = f(const_cast<double&>(d));
   }

This strips away the ``const-ness'' of d before passing it to f. You cannot use const_cast for any other types of casts, such as casting a base class pointer to a derived class pointer. If you do so, the compiler will report an error.