Here is a merged version of the C++ code for the first two answers.
#include <iostream>
#include <string>
using namespace std;
class Animal
{
public:
#ifdef VIRTUAL
virtual string says() { return "??"; }
#else
string says() { return "??"; }
#endif
};
class Dog: public Animal
{
public:
string says() { return "woof"; }
};
string func(Animal *a)
{
return a->says();
}
int main()
{
Animal *a = new Animal();
Dog *d = new Dog();
Animal *ad = d;
cout << "Animal a says\t\t" << a->says() << endl;
cout << "Dog d says\t\t" << d->says() << endl;
cout << "Animal dog ad says\t" << ad->says() << endl;
cout << "func(a) :\t\t" << func(a) << endl;
cout << "func(d) :\t\t" << func(d) << endl;
cout << "func(ad):\t\t" << func(ad)<< endl;
}
Two different results are:
Without #define virtual, it binds at compile time. Animal *ad and func(Animal *) all point to the Animal's says() method.
$ g++ virtual.cpp -o virtual
$ ./virtual
Animal a says ??
Dog d says woof
Animal dog ad says ??
func(a) : ??
func(d) : ??
func(ad): ??
With #define virtual, it binds at run time. Dog *d, Animal *ad and func(Animal *) point/refer to the Dog's says() method as Dog is their object type. Unless [Dog's says() "woof"] method is not defined, it will be the one searched first in the class tree, i.e. derived classes may override methods of their base classes [Animal's says()].
$ g++ virtual.cpp -D VIRTUAL -o virtual
$ ./virtual
Animal a says ??
Dog d says woof
Animal dog ad says woof
func(a) : ??
func(d) : woof
func(ad): woof
It is interesting to note that all class attributes (data and methods) in Python are effectively virtual. Since all objects are dynamically created at runtime, there is no type declaration or a need for keyword virtual. Below is Python's version of code:
class Animal:
def says(self):
return "??"
class Dog(Animal):
def says(self):
return "woof"
def func(a):
return a.says()
if __name__ == "__main__":
a = Animal()
d = Dog()
ad = d # dynamic typing by assignment
print("Animal a says\t\t{}".format(a.says()))
print("Dog d says\t\t{}".format(d.says()))
print("Animal dog ad says\t{}".format(ad.says()))
print("func(a) :\t\t{}".format(func(a)))
print("func(d) :\t\t{}".format(func(d)))
print("func(ad):\t\t{}".format(func(ad)))
The output is:
Animal a says ??
Dog d says woof
Animal dog ad says woof
func(a) : ??
func(d) : woof
func(ad): woof
which is identical to C++'s virtual define. Note that d and ad are two different pointer variables referring/pointing to the same Dog instance. The expression (ad is d) returns True and their values are the same <main.Dog object at 0xb79f72cc>.