I am learning c++ and am learning about the virtual keyword. I have scoured the internet trying to understand it to no avail. I went into my editor and did the following experiment, expecting it to print out the base message twice (because I was under the impression that the virtual keyword is needed to override functions). However, it printed out two different messages. Can someone explain to me why we need the virtual keyword if we can simply override functions and still seemingly get polymorphic behavior? Perhaps someone can help me and other people in the future understand virtual vs. overriding. (The output I am getting is "I am the base" followed by "I am the derived").
#include <iostream>
using namespace std;
class Base{
public:
void printMe(){
cout << "I am the base" << endl;
}
};
class Derived: public Base{
public:
void printMe(){
cout << "I am the derived" << endl;
}
};
int main() {
Base a;
Derived b;
a.printMe();
b.printMe();
return 0;
}
Consider the following example. The important line to illustrate the need for virtual and override is c->printMe();. Note that the type of c is Base*, however due to polymorphism it is correctly able to call the overridden method from the derived class. The override keyword allows the compiler to enforce that a derived class method matches the signature of a base class's method that is marked virtual. If the override keyword is added to a derived class function, that function does not also need the virtual keyword in the derived class as the virtual is implied.
#include <iostream>
class Base{
public:
virtual void printMe(){
std::cout << "I am the base" << std::endl;
}
};
class Derived: public Base{
public:
void printMe() override {
std::cout << "I am the derived" << std::endl;
}
};
int main() {
Base a;
Derived b;
a.printMe();
b.printMe();
Base* c = &b;
c->printMe();
return 0;
}
The output is
I am the base
I am the derived
I am the derived