Polymorphism implements runtime polymorphism through virtual functions, allowing the base class pointer to call the functions overwritten by the derived class; 1. Declare virtual functions (or pure virtual functions) in the base class so that the function can be rewritten; 2. The derived class rewrites virtual functions using the override keyword to achieve specific behavior; 3. Call functions using the base class pointer or reference to trigger dynamic binding; 4. The virtual destructor must be defined to ensure that the derived class objects are correctly destructed to prevent resource leakage, thereby fully implementing a safe and flexible polymorphic mechanism.
Polymorphism is one of the core features of C object-oriented programming. It allows the use of base class pointers or references to call functions of derived classes to implement "one interface, multiple behaviors". The most common way to implement it is through virtual functions .

Below is a simple C polymorphism example showing how to implement runtime polymorphism through virtual functions.
? Basic example: animal call polymorphic
#include <iostream> #include <vector> using namespace std; // Base class (abstract interface) class Animal { public: virtual void makeSound() const { cout << "Animal makes a sound" << endl; } // Virtual destructors are very important! Used to correctly release derived class object virtual ~Animal() = default; }; // Derived class: Dog class Dog: public Animal { public: void makeSound() const override { cout << "Woof! Woof!" << endl; } }; // Derived class: cat class Cat: public Animal { public: void makeSound() const override { cout << "Meow! Meow!" << endl; } }; // Derived class Bird : public Animal { public: void makeSound() const override { cout << "Chirp! Chirp!" << endl; } }; int main() { // Use base class pointers to store different types of animals vector<Animal*> animals = {new Dog(), new Cat(), new Bird()}; // Polymorphic call: same interface, different behaviors for (Animal* animal: animals) { animal->makeSound(); // Call makeSound() rewrites each } // Clean memory for (Animal* animal : animals) { delete animal; } return 0; }
? Output result:
Woof! Woof! Meow! Meow! Chirp! Chirp!
? Key points description:
-
virtual void makeSound()
: declared as a virtual function in the base class, indicating that this function can be rewritten in the derived class. -
override
: Make sure that the function really rewrites the virtual functions of the base class to improve code security. -
virtual ~Animal() = default;
: The virtual destructor is necessary , otherwise the destructor of the derived class will not be called when deleting the base class pointer, resulting in resource leakage. - The base class pointer (
Animal*
) can point to any derived class object. WhenmakeSound()
is called, the version of the corresponding object will be automatically called - this is runtime polymorphism .
? Go further: Pure virtual functions and abstract base classes
You can also define makeSound
as a pure virtual function , and the forced derived class must implement it:

class Animal { public: virtual void makeSound() const = 0; // Pure virtual function virtual ~Animal() = default; }; // In this way, Animal cannot be instantiated and can only be used as an interface
Then each derived class must implement makeSound()
, otherwise the compilation will report an error.
? Summary: The key steps to achieve polymorphism
- Declare virtual functions (or pure virtual functions) in the base class
- Rewrite functions using
override
in derived classes - Calling functions using base class pointers or references
- Remember to define the virtual destructor
Basically that's it. This example shows the most typical and practical usage of C polymorphism, which is suitable for beginners to understand and subsequent expansion (such as combining factory patterns, container management objects, etc.).

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