Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Thinking in C++
Prev Contents / Index Next

Variant return type

The Derived3 class above suggests that you cannot modify the return type of a virtual function during overriding. This is generally true, but there is a special case in which you can slightly modify the return type. If you’re returning a pointer or a reference to a base class, then the overridden version of the function may return a pointer or reference to a class derived from what the base returns. For example:

//: C15:VariantReturn.cpp
// Returning a pointer or reference to a derived
// type during ovverriding
#include <iostream>
#include <string>
using namespace std;

class PetFood {
  virtual string foodType() const = 0;

class Pet {
  virtual string type() const = 0;
  virtual PetFood* eats() = 0;

class Bird : public Pet {
  string type() const { return "Bird"; }
  class BirdFood : public PetFood {
    string foodType() const { 
      return "Bird food"; 
  // Upcast to base type:
  PetFood* eats() { return &bf; }
  BirdFood bf;

class Cat : public Pet {
  string type() const { return "Cat"; }
  class CatFood : public PetFood {
    string foodType() const { return "Birds"; }
  // Return exact type instead:
  CatFood* eats() { return &cf; }
  CatFood cf;

int main() {
  Bird b; 
  Cat c;
  Pet* p[] = { &b, &c, };
  for(int i = 0; i < sizeof p / sizeof *p; i++)
    cout << p[i]->type() << " eats "
         << p[i]->eats()->foodType() << endl;
  // Can return the exact type:
  Cat::CatFood* cf = c.eats();
  Bird::BirdFood* bf;
  // Cannot return the exact type:
//!  bf = b.eats();
  // Must downcast:
  bf = dynamic_cast<Bird::BirdFood*>(b.eats());
} ///:~

The Pet::eats( ) member function returns a pointer to a PetFood. In Bird, this member function is overloaded exactly as in the base class, including the return type. That is, Bird::eats( ) upcasts the BirdFood to a PetFood.

But in Cat, the return type of eats( ) is a pointer to CatFood, a type derived from PetFood. The fact that the return type is inherited from the return type of the base-class function is the only reason this compiles. That way, the contract is still fulfilled; eats( ) always returns a PetFood pointer.

If you think polymorphically, this doesn’t seem necessary. Why not just upcast all the return types to PetFood*, just as Bird::eats( ) did? This is typically a good solution, but at the end of main( ), you see the difference: Cat::eats( ) can return the exact type of PetFood, whereas the return value of Bird::eats( ) must be downcast to the exact type.

So being able to return the exact type is a little more general, and doesn’t lose the specific type information by automatically upcasting. However, returning the base type will generally solve your problems so this is a rather specialized feature.

Thinking in C++
Prev Contents / Index Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire