12. Polymorphism: Overriding

Override

Polymorphism: Overriding

"Overriding" a function occurs when:

  1. A base class declares a [ virtual function]( http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#glossary function).
  2. A derived class overrides that virtual function by defining its own implementation with an identical function signature (i.e. the same function name and argument types). 
class Animal {
public:
  virtual std::string Talk() const = 0;
};

class Cat {
public:
  std::string Talk() const { return std::string("Meow"); }
};

In this example, Animal exposes a virtual function: Talk() , but does not define it. Because Animal::Talk() is undefined, it is called a pure virtual function , as opposed to an ordinary (impure? 😉) virtual function .

Furthermore, because Animal contains a pure virtual function, the user cannot instantiate an object of type Animal . This makes Animal an abstract class .

Cat , however, inherits from Animal and overrides Animal::Talk() with Cat::Talk() , which is defined. Therefore, it is possible to instantiate an object of type Cat .

Instructions

  1. Create a class Dog to inherit from Animal .
  2. Define Dog::Talk() to override the virtual function Animal::Talk() .
  3. Confirm that the tests pass.

Workspace

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Workspace Information:

  • Default file path:
  • Workspace type: jupyter
  • Opened files (when workspace is loaded): n/a

Function Hiding

Function hiding is closely related, but distinct from , overriding.

A derived class hides a base class function, as opposed to overriding it, if the base class function is not specified to be virtual . 

class Cat { // Here, Cat does not derive from a base class
public:
  std::string Talk() const { return std::string("Meow"); }
};

class Lion : public Cat {
public:
  std::string Talk() const { return std::string("Roar"); }
};

In this example, Cat is the base class and Lion is the derived class. Both Cat and Lion have Talk() member functions.

When an object of type Lion calls Talk() , the object will run Lion::Talk() , not Cat::Talk() .

In this situation, Lion::Talk() is hiding Cat::Talk() . If Cat::Talk() were virtual , then Lion::Talk() would override Cat::Talk() , instead of hiding it. Overriding requires a virtual function in the base class.

The distinction between overriding and hiding is subtle and not terribly significant, but in certain situations hiding can lead to bizarre errors , particularly when the two functions have slightly different function signatures.

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