Item 24. Uses and Abuses of Inheritance

Difficulty: 6

"Why inherit?"

Inheritance is often overused, even by experienced developers. Always minimize coupling. If a class relationship can be expressed in more than one way, use the weakest relationship that's practical. Given that inheritance is nearly the strongest relationship you can express in C++ (second only to friendship), it's only really appropriate when there is no equivalent weaker alternative.

In this Item, the spotlight is on private inheritance, one real (if obscure) use for protected inheritance, and a recap of the proper use of public inheritance. Along the way, we will create a fairly complete list of all the usual and unusual reasons to use inheritance.

The following template provides list-management functions, including the ability to manipulate list elements at specific list locations.

// Example 1 
//
template <class T>
class MyList
{
public:
  bool   Insert( const T&, size_t index );
  T      Access( size_t index ) const;
  size_t Size() const;
private:
  T*     buf_;
  size_t bufsize_;
};

Consider the following code, which shows two ways to write a MySet class in terms of MyList. Assume that all important elements are shown.

// Example 1(a) 
//
template <class T>
class MySet1 : private MyList<T>
{
public:
  bool   Add( const T& ); // calls Insert()
  T      Get( size_t index ) const;
                          // calls Access()
  using MyList<T>::Size;
  //...
};

// Example 1(b)
//
template <class T>
class MySet2
{
public:
  bool   Add( const T& ); // calls impl_.Insert()
  T      Get( size_t index ) const;
                          // calls impl_.Access()
  size_t Size() const;    // calls impl_.Size();
  //...
private:
  MyList<T> impl_;
};

Give these alternatives some thought, and consider the following questions.

1. Is there any difference between MySet1 and MySet2?

2. More generally, what is the difference between nonpublic inheritance and containment? Make as comprehensive a list as you can of reasons why you would use inheritance instead of containment.

3. Which version of MySet would you prefer—MySet1 or MySet2?

4. Finally, make as comprehensive a list as you can of reasons why you would use public inheritance.