C++ Primer Plus

The plain type cast in C++ is also restricted. Basically, it can do anything the other type casts can do, plus some combinations, such as a static_cast or reinterpret_cast followed by a const_cast, but it can’t do anything else. Thus, the following type cast is allowed in C but, typically, not in C++ because for most C++ implementations the char type is too small to hold a pointer implementation:

char ch = char (&d);          // type cast #2 - convert address to a char

These restrictions make sense, but if you find such enforced good judgment oppressive, you still have C available.

Summary

Friends allow you to develop a more flexible interface for classes. A class can have other functions, other classes, and member functions of other classes as friends. In some cases, you may need to use forward declarations and to exert care in the ordering of class declarations and methods in order to get friends to mesh properly.

Nested classes are classes that are declared within other classes. Nested classes facilitate the design of helper classes that implement other classes but needn’t be part of a public interface.

The C++ exception mechanism provides a flexible way to deal with awkward programming events such as inappropriate values, failed I/O attempts, and the like. Throwing an exception terminates the function currently executing and transfers control to a matching catch block. catch blocks immediately follow a try block, and for an exception to be caught, the function call that directly or indirectly led to the exception must be in the try block. The program then executes the code in the catch block. This code may attempt to fix the problem, or it can terminate the program. A class can be designed with nested exception classes that can be thrown when problems specific to the class are detected. A function can include an exception specification that identifies the exceptions that can be thrown in that function, although C++11 deprecates that feature. Uncaught exceptions (those with no matching catch block) by default terminate a program. So do unexpected exceptions (those not matching an exception specification.)

The RTTI features allow a program to detect the type of an object. The dynamic_cast operator is used to cast a derived-class pointer to a base-class pointer; its main purpose is to ensure that it’s okay to invoke a virtual function call. The typeid operator returns a type_info object. Two typeid return values can be compared to determine whether an object is of a specific type, and the returned type_info object can be used to obtain information about an object.

The dynamic_cast, static_cast, const_cast, and reinterpret_cast operators provide safer, better-documented type casts than the general type cast mechanism.

Chapter Review

1. What’s wrong with the following attempts at establishing friends?

a.

class snap {

    friend clasp;

     ...

};

class clasp { ... };

b.

class cuff {

public:

      void snip(muff &) { ... }

      ...

};

class muff {

     friend void cuff::snip(muff &);

     ...

};

c. class muff {

      friend void cuff::snip(muff &);

     ...

};

class cuff {

public:

      void snip(muff &) { ... }

      ...

};

2. You’ve seen how to create mutual class friends. Can you create a more restricted form of friendship in which only some members of Class B are friends to Class A and some members of A are friends to B? Explain.

3. What problems might the following nested class declaration have?

class Ribs

{

private:

    class Sauce

    {

          int soy;

          int sugar;

   public:

          Sauce(int s1, int s2) : soy(s1), sugar(s2) { }

   };

   ...

};

4. How does throw differ from return?

5. Suppose you have a hierarchy of exception classes that are derived from a base exception class. In what order should you place catch blocks?

6. Consider the Grand, Superb, and Magnificent classes defined in this chapter. Suppose pg is a type Grand * pointer that is assigned the address of an object of one of these three classes and that ps is a type Superb * pointer. What is the difference in how the following two code samples behave?

if (ps = dynamic_cast(pg))

    ps->say();  // sample #1

if (typeid(*pg) == typeid(Superb))

    (Superb *) pg)->say();  // sample #2

7. How is the static_cast operator different from the dynamic_cast operator?

Programming Exercises

1. Modify the Tv and Remote classes as follows:

a. Make them mutual friends.

b. Add a state variable member to the Remote class that describes whether the remote control is in normal or interactive mode.

c. Add a Remote method that displays the mode.

d. Provide the Tv class with a method for toggling the new Remote member. This method should work only if the TV is in the on state.

Write a short program that tests these new features.