Implementation

The implementation of the Wrapper<T> class member functions is shown in Program . The Wrapper<T> class default constructor simply initializes the member variable datum using its default constructor. A second constructor takes as its lone argument a const reference to an object of type T and copies that value to the member variable datum. In effect, the constructor wraps its argument.

  
Program: Wrapper<T> Class Member Function Definitions

A type-cast operator is defined, operator T const&, which converts from an object of type Wrapper<T> to a const reference to an object of type T. In effect, the type-cast operator unwraps the contained object! This, together with the automatic type coercion rules of C++, makes it possible to use the wrapped object in the same context in which the unwrapped object is expected.

The Hash member function of the Wrapper<T> class simply calls the global (non-member) function, ::Hash(T). In effect, we have punted on the implementation of the Hash function. The implementation of a suitable hash function for an object of type T depends on the actual type used. However, the Wrapper<T> class is generic--we don't know what T is. Hash functions are discussed in Chapter .

The CompareTo function takes as its lone argument a const reference to Object. It uses the dynamiccast  operator to convert the argument to the type Wrapper<T>. In C++, the dynamic cast will succeed only if the type of the referenced actually is Wrapper<T>. Otherwise a badcast  exception is thrown. The CompareTo member function is implemented in the same way as the Hash member function. I.e., it calls the global (non-member) function ::Compare to do the actual comparison.

The implementation of the Put member function is trivial. It simply inserts datum into the given ostream using operator<<. Again, this assumes that the operator has already been defined for objects of type T.

Program  uses the Wrapper<T> class defined above to define the Int, Char, Double, and String object classes which simply encapsulate variables of type int, char, double, and string, respectively.

  
Program: Int, Char, Double and String Class Definitions

These declarations require the existence of suitable ::Compare and ::Hash functions. E.g., the Int class definition requires the existence of a Compare(int) and a Hash(int) function. Hash functions are described in Chapter . The implementation of a suitable compare function is left as a project for the reader (Project ).

Copyright © 1997 by Bruno R. Preiss, P.Eng. All rights reserved.