Virtual Table in C++
Introduction to Virtual Table:
The virtual table, a unique kind of late binding, implements virtual functions in C++. In order to resolve function calls in a dynamically binding manner, the virtual table acts as a lookup table of functions. Other terms for the virtual table include “vtable”, “virtual method table”, “virtual function table”, and “dispatch table”. Here, the compiler will create the virtual table separately for each class.
Though it's a little difficult to explain in words, the virtual table is fairly straight forward. Initially, every class that utilizes virtual functions (or descended from a class that uses virtual functions) is first given its virtual table. Simply put, the compiler creates a static array for this table during compilation. For every virtual function that class objects may call, there is an entry in a virtual table. The most derived function accessible by that class is indicated by each entry in the table, which is just a function pointer.
The base class additionally receives a hidden pointer from the compiler that we will refer to as *__vptr (virtual table pointer). When a class instance is generated, *__vptr is immediately set (automatically) to point to the virtual table for that class. The *__vptr pointer is a true pointer, unlike the *this pointer, which is generally a function parameter utilized by the compiler to determine self-references. As a result, it increases the size of each class object allocated by one pointer. Additionally, it implies that derived classes inherit *__vptr, which is significant. By presenting the idea of a virtual table, C++ enables developers to carry out runtime polymorphism; in this way, it upgrades the code reusability and adaptability.
Role of Virtual Tables:
The idea of a virtual table is relevant when we are working with virtual functions. There is a virtual table related to each class that has at least one virtual function. Pointers to the virtual functions of that class are contained in this table. To make sure the right function is executed when a virtual function is called, the program searches for the function's address in the virtual table linked to the object's actual type.
How to Get Dynamic Binding for Virtual Tables:
A major benefit of using virtual tables and virtual functions is dynamic binding. Based on the type of the object, it enables the program to decide which appropriate function to call during runtime. Static binding, in contrast, resolves the function call at compile time.
OOPs in Virtual Tables:
The usage of virtual tables and virtual functions in conjunction with Object Oriented Programming (OOPs) offers a strong framework for developing adaptable and extendable applications. Developers may create complicated, robust, and maintainable systems by utilizing ideas like inheritance and polymorphism.
Polymorphism in Virtual Tables:
One of Object-Oriented programming's (OOPs) four guiding concepts is polymorphism. It enables objects to execute the methods of their specific derived types while still being viewed as instances of their base types. As a result, code may be created that is flexible and extensible and can function with many different kinds that are connected.
Polymorphism mostly comes in two forms:
- Compile-time (Static) Polymorphism: Techniques like function overloading and templates are used to achieve this. Based on the kind of arguments and how well they match function signatures, the specific function to be called is chosen at compile time.
- Runtime Polymorphism (Dynamic Binding): Virtual functions and virtual tables make this easier. The actual type of the object determines the specific function that will be executed at runtime.
Virtual Functions:
Virtual functions, sometimes referred to as virtual methods, are member functions that are specified within a base class but are redefined (overridden) by the derived class. When you use a pointer or a reference to the base class to refer to an object of a derived class, you can call a virtual function for that object and run the derived class's version of the method. No matter the type of reference (or pointer) used for the function call, virtual functions make sure the right function is called for an object. Their primary purpose is to implement runtime polymorphism. A base class's virtual keyword is used to declare functions. A function call is resolved during its runtime.
Rules for Virtual Functions:
The guidelines for C++ virtual functions are as follows:
- Virtual functions can never be static.
- A friend function of a different class can be a virtual function.
- Runtime polymorphism should be achieved by accessing virtual functions via a pointer or reference of base class type.
- Both the base class and the derived class should share the same prototype for virtual functions.
- They are always specified in the base class and replaced in derived classes. The function from the base class is used if the derived class does not override or redefine the virtual function, which is optional.
- It is possible for a class to have a virtual destructor but not a virtual constructor.
The syntax for declaring Virtual Functions:
Unquestionably! In C++, characterizing virtual functions and their cooperation with virtual tables includes explicit language structure (syntax). Here is a breakdown:
Syntax:
class base {
public:
virtual return_type function_name (parameters) {
// Function execution in base class
}
// Initialize the derived class
};
Explanation:
- virtual: This keyword demonstrates that the function is virtual. It comes first in the function declaration after the return type.
- return_type: The data type that the function returns is known as return_type.
- function_name: The virtual function's name is given here.
- parameters: These are the function's accepted input parameters (if any).
- { }: This block includes the base class's implementation of the function.
Example:
Program:
#include <iostream>
using namespace std;
class Base {
public:
virtual void show() {
cout << "Printing the Base class\n";
}
};
class Derived : public Base {
public:
void show() override {
cout << "Printing the Derived class\n";
}
};
int main() {
Base* ptr;
Base baseObj;
Derived derivedObj;
ptr = &baseObj;
ptr->show();
ptr = &derivedObj;
ptr->show();
return 0;
}
Output:
![Virtual Table in C++](https://static.tutorialandexample.com/cpp/virtual-table-in-cpp1.png)
Explanation:
- With the virtual function show(), we define the base class named Base. The intention is for derived classes to replace this function.
- Next, we develop a derived class. Which overrides the show() function and is derived from the Base class.
- Both Base and Derived class objects are created in the main() function.
- To point the objects from both classes, we use a pointer ptr of type Base*.
- The program uses dynamic dispatch to decide which adaption of the function to execute when we call “ptr --> show()”. If show() hadn't been virtual, the type of the pointer would have determined how it was resolved at compilation time.
- We characterize a base class Base with a virtual capability show(). This capability is intended to be abrogated by inferred classes.
- The result exhibits that despite the fact that ptr is of type Base*, it accurately calls the overridden version of show() in the Determined (Derived) class when ptr focuses on the derived object.
- This example features how virtual tables and virtual functions cooperate to accomplish runtime polymorphism in C++ programming.
Virtual Functions Operations (VPTR and VTABLE):
On the off chance that an object of that class is made, a virtual pointer (VPTR) is embedded as a data member from the class to highlight the VTABLE of that class. For each new object made, another virtual pointer is embedded as a data member from that class.
Regardless of whether the object is made, the class contains, as a member, VTABLE, a static array of function pointers. Each virtual function in that class is stored at its address in a cell of this table.
![Virtual Table in C++](https://static.tutorialandexample.com/cpp/virtual-table-in-cpp2.png)
Example:
Program:
#include <iostream>
using namespace std;
class base {
public:
void function_1() { cout << "base function 1\n"; }
virtual void function_2() { cout << "base function 2\n"; }
virtual void function_3() { cout << "base function 3\n"; }
virtual void function_4() { cout << "base function 4\n"; }
};
class derived : public base {
public:
void function_1() { cout << "derived function 1\n"; }
void function_2() { cout << "derived function 2\n"; }
void function_4(int x) { cout << "derived function 4\n"; }
};
int main()
{
base* p;
derived obj1;
p = &obj1;
p->function_1();
p->function_2();
p->function_3();
p->function_4();
return 0;
}
Output:
![Virtual Table in C++](https://static.tutorialandexample.com/cpp/virtual-table-in-cpp3.png)
Explanation:
At first, we make a pointer of the sort base class and instate it with the location of the inferred class object. At the point when we make an object of the determined class, the compiler makes a pointer as an information individual from the class containing the location of VTABLE of the inferred class.
Similar to the example given above, late and early binding is used. Function_1() calls the base class version of the function, function_2() calls the derived class version because it is overridden in the derived class, function_3() calls the base class version because it is not overridden in the derived class and is a virtual function, and function_4() calls the base class version because it is not overridden.
Limitations of Virtual Functions:
The function call needs somewhat longer because of the virtual component and makes it more challenging for the compiler to improve in light of the fact that it doesn't know precisely what function will be called at compile time.
In a complicated framework, virtual function can make it somewhat more challenging to sort out where a function is being called from.
Real World usage of Virtual Functions:
- Graphical User Interface (GUI) frameworks
- Database Interfaces
- Game Development
- Finance Applications
- Simulation & Modelling
- Plugin Systems
- Artificial Intelligence & Machine Learning
- Multimedia Processing
- Networking & Communication Protocols
- Robotics & Automation
Advantages of Virtual Tables:
Virtual tables (vtables) give a few benefits in C++ programming, especially with regard to accomplishing polymorphism and dynamic binding. Here are a portion of the critical benefits of utilizing vtables:
Accomplishing Runtime Polymorphism: Vtables permit objects of various derived classes to be dealt with consistently through a base class pointer or reference. This empowers a polymorphic way of behaving, where the proper function is called at runtime in light of the actual type of the object.
Dynamic Binding: The choice of which function to call with vtables is done at runtime rather than during compilation. As a result, dynamic binding is possible, where the appropriate function is chosen during program execution based on the type of the object.
Adaptability in Object Behavior: Vtables give an instrument to objects to change their way of behaving at runtime. This is especially helpful in situations where the way of behaving of objects should be resolved dynamically founded on client input or other runtime conditions.
Simplified Code and Maintenance: By utilizing virtual tables and virtual functions, designers can compose more viable and extensible code. While adding new determined classes, is not unexpected, it's simply important to carry out the particular way of behaving for that class rather than adjusting existing code.
Encapsulation and Abstraction: Vtables take into consideration a serious level of abstraction and encapsulation. The point of interaction given by the base class through virtual functions conceals the execution subtleties of determined classes, taking into consideration a cleaner detachment of concerns.
Decreased Code Duplication: Through virtual functions and inheritance, a normal way of behaving can be characterized in a base class. This diminishes the need to copy code across various derived classes, prompting more brief and viable code.
More straightforward Unit Testing and Mocking: Vtables make mocking and unit testing in object-oriented code easier. In derived classes, virtual functions can be changed to provide mock implementations for testing.
Interoperability with Frameworks and Libraries: Numerous libraries and systems (frameworks) in C++ influence polymorphism and virtual functions. Understanding and utilizing vtables empowers engineers to work really with outsider code and incorporate it consistently into their ventures.
Improved Extensibility and Versatility: Vtables make it simpler to broaden the usefulness of a codebase. Newly determined classes can be added without huge changes to existing code, making the framework more versatile.
Further developed Design Pattern Utilization: Polymorphism is a key component of design patterns like the State Pattern, Strategy Pattern, and others, and virtual tables make it easier. Applying these patterns is improved by using virtual tables.
Hence, we can say that the virtual tables assume a significant part in empowering polymorphic ways of behaving, dynamic binding, and keeping up with spotless, secluded code in object-oriented C++ programming language. They upgrade the adaptability, viability, and extensibility of programming frameworks.
Conclusion:
Hence, we can say that the virtual tables in C++ comprise a key system for accomplishing dynamic binding and polymorphism in object-oriented programming. They empower objects of various determined classes to be dealt with consistently through a typical base class interface, working with adaptability and extensibility in the code plan.
By using virtual functions, designers can make ordered hierarchies of classes that show different ways of behaving in light of their actual types. This advances code reusability and improves support, as alterations in determined classes don't require changes to existing code.
Understanding the fundamental v-table instrument is pivotal for accomplished C++ programming. It enables engineers to tackle the maximum capacity of polymorphism, considering versatile and adaptable programming arrangements.
In any case, it's basic to practice alerts and smart plans while executing virtual functions. Finding some kind of harmony between inheritance hierarchies, giving clear points of interfaces, and keeping away from excessive above is fundamental for productive and viable code.
In real-world applications, virtual tables find broad use in areas going from GUI structures to game development, simulation modelling and database interference. Their flexibility makes them a foundation of present-day computer programming, empowering the formation of vigorous, versatile, and measured frameworks.
Basically, virtual tables epitomize the force of object-oriented programming, giving an instrument that rises above static typing and enables designers to fabricate complex, unique applications. Excelling at virtual tables makes the way for a universe of adaptable and expressive programming in C++.