Can a C++ function be declared virtual? What is the role of virtual functions?

Virtual functions in C allow derived classes to redefine methods inherited from base classes to achieve polymorphism. The syntax is: declare a virtual function with the virtual keyword in the base class, and redefine it with override in the derived class. By calling a virtual function through a pointer or reference, a derived class object can call a base class virtual function. The main functions of virtual functions include: realizing polymorphism, supporting dynamic binding and providing abstraction.

Virtual function in C

Introduction

Virtual function is a A special type of member function that allows a derived class to redefine methods inherited from a base class. This enables polymorphism, i.e. derived class objects can be treated in the same way as base class objects.

Syntax

Virtual functions are declared in the base class, using the virtual keyword:

class Base {
public:
  virtual void func() { /* ... */ }
};

In the derived class, Virtual functions can be redefined:

class Derived : public Base {
public:
  void func() override { /* ... */ }
};

Virtual function calls are completed through pointers or references, so derived class objects can call virtual functions in the parent class:

Base* base = new Derived;
base->func(); // 调用 Derived::func()

Practical case

Consider the following example:

class Shape {
public:
  virtual double area() const = 0;
};

class Circle : public Shape {
public:
  Circle(double radius) : _radius(radius) {}
  double area() const override { return _radius * _radius * 3.14; }
  double _radius;
};

class Square : public Shape {
public:
  Square(double side) : _side(side) {}
  double area() const override { return _side * _side; }
  double _side;
};

int main() {
  Shape* shapes[] = {new Circle(5), new Square(4)};
  double total_area = 0;
  for (Shape* shape : shapes) {
    total_area += shape->area();
  }
  std::cout << "Total area: " << total_area << std::endl;
}

In this example, the base class Shape defines an abstract function area(), and the derived class Circle and Square provide their own implementations. The main function creates an array of Shape pointers pointing to objects of the derived class, and calculates the total area through virtual function calls.

Function

The main functions of virtual functions include:

• Realize polymorphism:Allow derived class objects to Generic way to interact with base class objects.
• Support dynamic binding: Bind function calls at runtime and call derived class methods according to the actual object type.
• Provide abstraction: The base class can define abstract methods that are specifically implemented by the derived class.

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