A virtual base class is a class that can be inherited by other classes, but its constructor is not called during the initialization of the derived classes. In other words, a virtual base class is not supposed to have any objects created directly from it. Instead, it is supposed to be used as a common base class for other classes.
In this article, we will discuss virtual base classes in detail and provide code examples to illustrate their usage.
What is a Virtual Base Class?
A virtual base class is a class that serves as a common base class for multiple derived classes. It is called “virtual” because its constructor is not called during the initialization of the derived classes. Instead, it is called only once during the initialization of the most derived class.
Here is an example:
class Base {
public:
Base() {}
void fun() {}
};
class Derived1 : public Base {
};
class Derived2 : public Base {
};
class Derived3 : public Derived1, public Derived2 {
};
In this example, the Base class is a virtual base class for both Derived1 and Derived2. The Derived3 class inherits from both Derived1 and Derived2, which both inherit from Base.
When we create an object of the Derived3 class, the constructor of the Base class is called only once during the initialization of the most derived class. This prevents multiple instances of the Base class from being created.
Why Use a Virtual Base Class?
A virtual base class is useful when there are multiple paths from a derived class to a common base class. For example, consider a class hierarchy where Shape is the common base class for Circle, Rectangle, and Triangle.
class Shape {
public:
virtual void draw() = 0;
};
class Circle : public Shape {
public:
void draw() {}
};
class Rectangle : public Shape {
public:
void draw() {}
};
class Triangle : public Shape {
public:
void draw() {}
};
Now, consider a class hierarchy where ColoredShape is the common base class for RedCircle, GreenRectangle, and BlueTriangle.
class ColoredShape : public virtual Shape {
public:
virtual void setColor() = 0;
};
class RedCircle : public ColoredShape, public Circle {
public:
void setColor() {}
};
class GreenRectangle : public ColoredShape, public Rectangle {
public:
void setColor() {}
};
class BlueTriangle : public ColoredShape, public Triangle {
public:
void setColor() {}
};
In this example, the Shape class is a common base class for Circle, Rectangle, and Triangle, and the ColoredShape class is a common base class for RedCircle, GreenRectangle, and BlueTriangle.
Because we want to ensure that there is only one instance of the Shape class, we derive ColoredShape from Shape virtually. This ensures that the Shape constructor is called only once during the initialization of the most derived class.
Code Examples
Let’s take a closer look at how virtual base classes work in C++ by examining some code examples.
Example 1: Circle and Rectangle Classes
class Shape {
public:
Shape() {}
virtual double getArea() = 0;
};
class ColoredShape : public virtual Shape {
public:
ColoredShape() {}
virtual void setColor() = 0;
};
class Circle : public virtual Shape {
public:
Circle(double r) : radius(r) {}
virtual double getArea() {
return 3.14159 * radius * radius;
}
protected:
double radius;
};
class Rectangle : public virtual Shape {
public:
Rectangle(double w, double h) : width(w), height(h) {}
virtual double getArea() {
return width * height;
}
protected:
double width, height;
};
class ColoredCircle : public ColoredShape, public Circle {
public:
ColoredCircle(double r) : Circle(r) {}
virtual void setColor() {}
};
int main() {
ColoredCircle cc(5);
cc.getArea();
return 0;
}
In this example, we create a base class Shape that has a pure virtual function getArea(). We also create a class ColoredShape that inherits from Shape virtually and has a pure virtual function setColor().
We then create two classes Circle and Rectangle that inherit from Shape virtually and implement the getArea() function. Circle has a radius data member and Rectangle has width and height data members.
Finally, we create a class ColoredCircle that inherits from both ColoredShape and Circle. This class implements the setColor() function and initializes the radius data member.
In the main function, we create an object of the ColoredCircle class and call its getArea() function.
Example 2: Diamond Class Hierarchy
class A {
public:
A() {}
virtual void methodA() = 0;
};
class B : public virtual A {
public:
B() {}
virtual void methodB() = 0;
};
class C : public virtual A {
public:
C() {}
virtual void methodC() = 0;
};
class D : public B, public C {
public:
D() {}
virtual void methodA() {}
virtual void methodB() {}
virtual void methodC() {}
};
int main() {
D d;
d.methodA();
return 0;
}
In this example, we create a base class A with a pure virtual function methodA(). We then create two classes B and C that inherit from A virtually and have pure virtual functions methodB() and methodC(), respectively.
We then create a class D that inherits from both B and C. The D class implements all three functions methodA(), methodB(), and methodC().
In the main function, we create an object of the D class and call its methodA() function. This is an example of a diamond class hierarchy, where there are multiple paths from D to A.
Conclusion
In conclusion, a virtual base class is a class that serves as a common base class for multiple derived classes, but its constructor is not called during the initialization of the derived classes. This prevents multiple instances of the virtual base class from being created.
Virtual base classes are useful when there are multiple paths from a derived class to a common base class. They ensure that there is only one instance of the common base class. We have provided examples of using virtual base classes to implement shape classes and a diamond class hierarchy.
Understanding virtual base classes is an important part of object-oriented programming in C++. We hope that this article has helped you to understand virtual base classes and their usage in C++.
I can provide more information on the topics covered in the previous article. Here are some additional details regarding virtual base classes and the examples provided.
Virtual Base Classes
Virtual base classes are classes that are designed to serve as a common base class for multiple derived classes. The key difference between a virtual base class and a regular base class is that the constructor for a virtual base class is not called during the initialization of the derived classes.
This is done to ensure that there is only one instance of the virtual base class in memory, even if it is inherited by multiple classes. When a class inherits from a virtual base class, the virtual base class is added to the inherited classes list only once, regardless of how many times it appears in the class hierarchy.
Virtual base classes are declared by adding the keyword "virtual" before the access specifier in the class definition. Here is an example:
class VirtualBase {
public:
VirtualBase() {}
virtual void doSomething() = 0;
};
class Derived1 : public virtual VirtualBase {
public:
Derived1() {}
void doSomething() override {}
};
class Derived2 : public virtual VirtualBase {
public:
Derived2() {}
void doSomething() override {}
};
class SomeOtherClass : public Derived1, public Derived2 {
public:
SomeOtherClass() {}
void doSomething() override {}
};
In this example, VirtualBase is a virtual base class for both Derived1 and Derived2. When SomeOtherClass inherits from Derived1 and Derived2, the virtual base VirtualBase is only added once to the hierarchy.
The SomeOtherClass class has its own implementation of the doSomething() function, which overrides any implementation in the derived classes.
Example 1: Circle and Rectangle Classes
In the example provided in the previous article, we created base classes Shape and ColoredShape, and derived classes Circle and Rectangle. We then created a class ColoredCircle that inherited from both ColoredShape and Circle.
The ColoredShape class inherited from Shape virtually, ensuring that there was only one instance of Shape in the hierarchy. This class also provided a pure virtual function setColor() that was implemented in the ColoredCircle class.
The Circle and Rectangle classes both provided an implementation of the getArea() function, which was a pure virtual function declared in the Shape class. The ColoredCircle class did not need to provide its own implementation of getArea(), as it was already implemented in the Circle class.
Example 2: Diamond Class Hierarchy
In the example provided in the previous article, we created a diamond class hierarchy with classes A, B, C, and D. The B class and C class both inherited from A virtually, ensuring that there was only one instance of A in the hierarchy.
The D class inherited from both B and C, and implemented all three virtual functions, methodA(), methodB(), and methodC().
The main() function created an object of the D class and called its implementation of methodA(). Because D inherited A virtually through both B and C, there was only one instance of A in the hierarchy and only one implementation of methodA() to be called.
Conclusion
Virtual base classes are an important feature in C++ that allow for multiple inheritance with a common base class. By inheriting from a virtual base class, you can ensure that there is only one instance of the base class in memory, even if it is inherited by multiple derived classes.
The examples provided in the previous article demonstrated how virtual base classes can be used to create shape classes and diamond class hierarchies. By understanding the usage and implementation of virtual base classes, you can create more flexible and efficient class hierarchies in your C++ code.
Popular questions
Sure, here are 5 questions about virtual base classes with answers:
-
What is a virtual base class?
A virtual base class is a class that serves as a common base class for multiple derived classes, but its constructor is not called during the initialization of the derived classes. This ensures that there is only one instance of the virtual base class in memory. -
What is the purpose of using a virtual base class?
The purpose of using a virtual base class is to avoid multiple inheritance ambiguity. It ensures that there is only one instance of the base class in memory, even if it is inherited by multiple derived classes. -
How do you declare a virtual base class in C++?
You declare a virtual base class in C++ by adding the keyword "virtual" before the access specifier in the class definition. For example:class Derived : public virtual Base. -
Can a derived class inherit from a virtual base class as well as a non-virtual base class?
Yes, a derived class can inherit from both a virtual base class and a non-virtual base class. However, if the same virtual base class is inherited virtually and non-virtually, it can cause ambiguity. -
What is a diamond class hierarchy, and how are virtual base classes used to resolve it?
A diamond class hierarchy occurs when a class inherits from two classes that inherit from the same base class. This can cause ambiguity when both inherited classes have the same member function name. Virtual base classes resolve this by ensuring that there is only one instance of the base class in memory. This is done by inheriting the base class virtually in the intermediate classes, so that it is only added once to the inheritance hierarchy.
Tag
Hybrid
