[19] Inheritance — basics
(Part of C++ FAQ Lite, Copyright © 1991-2000, Marshall Cline, cline@parashift.com)
FAQs in section [19]:
- [19.1] Is inheritance important to C++?
- [19.2] When would I use inheritance?
- [19.3] How do you express inheritance in C++?
- [19.4] Is it OK to convert a pointer from a derived class to its base class?
- [19.5] What's the difference between public:, private:, and protected:?
- [19.6] Why can't my derived class access private: things from my base class?
- [19.7] How can I protect derived classes from breaking when I
change internal parts?
[19.1] Is inheritance important to C++?
Yep.
Inheritance is what separates abstract data type (ADT) programming from OO programming.
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[19.2] When would I use inheritance?
As a specification device.
Human beings abstract things on two dimensions: part-of and kind-of. A Ford Taurus is-a-kind-of-a Car, and a Ford Taurus has-a Engine, Tires, etc. The part-of hierarchy has been a part of software since the ADT style became relevant; inheritance adds "the other" major dimension of decomposition.
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[19.3] How do you express inheritance in C++?
By the : public syntax:
public:
// ...
};
We state the above relationship in several ways:
- Car is "a kind of a" Vehicle
- Car is "derived from" Vehicle
- Car is "a specialized" Vehicle
- Car is a "subclass" of Vehicle
- Car is a "derived class" of Vehicle
- Vehicle is the "base class" of Car
- Vehicle is the "superclass" of Car (this not as common in the C++ community)
(Note: this FAQ has to do with public inheritance; private and protected inheritance are different.)
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[19.4] Is it OK to convert a pointer from a derived class to its base class?
Yes.
An object of a derived class is a kind of the base class. Therefore the conversion from a derived class pointer to a base class pointer is perfectly safe, and happens all the time. For example, if I am pointing at a car, I am in fact pointing at a vehicle, so converting a Car* to a Vehicle* is perfectly safe and normal:
void g(Car* c) { f(c); } // Perfectly safe; no cast
(Note: this FAQ has to do with public inheritance; private and protected inheritance are different.)
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[19.5] What's the difference between public:, private:, and protected:?
- A member (either data member or member function) declared in a private: section of a class can only be accessed by member functions and friends of that class
- A member (either data member or member function) declared in a protected: section of a class can only be accessed by member functions and friends of that class, and by member functions and friends of derived classes
- A member (either data member or member function) declared in a public: section of a class can be accessed by anyone
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[19.6] Why can't my derived class access private: things from my base class?
To protect you from future changes to the base class.
Derived classes do not get access to private members of a base class. This effectively "seals off" the derived class from any changes made to the private members of the base class.
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[19.7] How can I protect derived classes from breaking when I
change internal parts?
A class has two distinct interfaces for two distinct sets of clients:
- It has a public: interface that serves unrelated classes
- It has a protected: interface that serves derived classes
Unless you expect all your derived classes to be built by your own team, you should consider making your base class's bits be private:, and use protected: inline access functions by which derived classes will access the private data in the base class. This way the private bits can change, but the derived class's code won't break unless you change the protected access functions.
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Revised Jul 10, 2000