Virtual functions are a fundamental concept in C++ programming, particularly in the realm of object-oriented programming (OOP). They enable polymorphism, a core principle of OOP, allowing functions to behave differently based on the object that invokes them. Understanding virtual functions is crucial for any programmer looking to leverage the full potential of C++ in creating flexible and scalable software designs.
Core Concepts of Virtual Functions
Definition and Syntax
Virtual functions in C++ are member functions of a class that can be overridden in its derived classes. Declaring a function as virtual signals to the compiler that we expect the function to be overridden in a subclass. The syntax for declaring a virtual function is straightforward:
class Base {
public:
virtual void display();
};
How Virtual Functions Work
When a virtual function is called through a pointer or a reference to the base class, the C++ runtime determines which function to invoke based on the type of the object pointed to, not the type of the pointer. This dynamic linkage is what makes polymorphism possible in C++.
Implementing Virtual Functions
Overriding Virtual Functions
A derived class overrides a virtual function by simply defining a function with the same signature. This overridden function replaces the base class version when called through a base class reference or pointer.
class Derived : public Base {
public:
void display() override;
};
The Virtual Destructor
One of the key aspects of virtual functions is the virtual destructor. It ensures that when an object of a derived class is deleted through a pointer of the base class type, the appropriate destructor is called, avoiding resource leaks or undefined behavior.
class Base {
public:
virtual ~Base();
};
Best Practices and Common Uses
Ensuring Polymorphic Behavior
Always use virtual functions when you want a member function to exhibit polymorphic behavior. This is particularly important in frameworks and libraries where extensibility is a key feature.
Abstract Classes and Pure Virtual Functions
In some cases, you might want to create classes that are meant to be base classes only. This can be achieved using pure virtual functions, which are declared by assigning 0 to the virtual function:
class AbstractClass {
public:
virtual void pureVirtualFunction() = 0;
};
by the Bito team. 
