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C++ function override

Function overriding means a derived class provides its own version of a virtual function from a base class.

The base class defines the interface.

The derived class defines the specialized behavior.

Simple example

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#include <iostream>

class Animal {
public:
    virtual void speak() const
    {
        std::cout << "animal sound\n";
    }
};

class Dog : public Animal {
public:
    void speak() const override
    {
        std::cout << "woof\n";
    }
};

void make_sound(const Animal& animal)
{
    animal.speak();
}

int main()
{
    Dog dog;
    make_sound(dog);
}

Output:

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woof

make_sound() receives an Animal&, but the real object is a Dog.

Because speak() is virtual, C++ calls Dog::speak().

Why use override

Use override when a derived class replaces a virtual function.

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void speak() const override

override tells the compiler:

this function must override a virtual function from the base class.

This is important because the function signature must match exactly.

Common mistake

The base function is const:

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class Animal {
public:
    virtual void speak() const
    {
        std::cout << "animal sound\n";
    }
};

But the derived function forgets const:

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class Dog : public Animal {
public:
    void speak() override // error
    {
        std::cout << "woof\n";
    }
};

This does not match the base function.

The correct override is:

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class Dog : public Animal {
public:
    void speak() const override
    {
        std::cout << "woof\n";
    }
};

override catches this mistake at compile time.

Without override, the compiler may treat the derived function as a new function instead of an override.

Virtual and override rules

Use these rules:

Rule Meaning
Put virtual in the base class enables runtime polymorphism
Put override in the derived class asks the compiler to check the override
override does not make a function virtual the base function must already be virtual
The function signature must match same name, parameters, const, and qualifiers
Use a virtual destructor in polymorphic base classes allows safe deletion through a base pointer

Example:

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class Animal {
public:
    virtual void speak() const = 0;
    virtual ~Animal() = default;
};

class Dog : public Animal {
public:
    void speak() const override
    {
    }
};

Wrong example:

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class Animal {
public:
    void speak() const
    {
    }
};

class Dog : public Animal {
public:
    void speak() const override // error: Animal::speak() is not virtual
    {
    }
};

override is a safety check.

It does not replace virtual.

Key point

Always use override when overriding a virtual function.

It makes mistakes visible early, especially when you accidentally change:

  • const
  • function parameters
  • return type
  • reference qualifiers

Pure virtual function

A pure virtual function has = 0.

It means the base class does not provide normal behavior.

Derived classes must implement the function.

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#include <iostream>

class Animal {
public:
    virtual void speak() const = 0;
};

class Dog : public Animal {
public:
    void speak() const override
    {
        std::cout << "woof\n";
    }
};

class Cat : public Animal {
public:
    void speak() const override
    {
        std::cout << "meow\n";
    }
};

void make_sound(const Animal& animal)
{
    animal.speak();
}

int main()
{
    Dog dog;
    Cat cat;

    make_sound(dog);
    make_sound(cat);
}

You cannot create an object directly from a class that has a pure virtual function:

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Animal animal; // error: Animal is abstract

Use a pure virtual function when the base class should define the interface, but each derived class must define its own behavior.