Difference Between Constructor & Destructor In C++ : 10 Key Points

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Constructors and destructors are special types of member functions integral to the concept of classes and, hence, object-oriented programming (OOPs) in C++ and other programming languages. While these special methods have the same name as the class and are used to deal with class objects, one must understand the multiple differences between constructor and destructor in C++ to write effective code.

In this article, we will explore the difference between constructor and destructor in C++ programming language. At a basic level, the primary difference between a constructor and a destructor is their main purpose, i.e., a constructor is used to initialize the object. Whereas a destructor is used to clean up after an object has been created. 

What Is A Constructor In C++?

As mentioned before, a constructor is a special class function/ member function that is called when an instance of a class is created. Its primary purpose is to initialize the attributes of the object/ data members.

• If the programmer does not provide an explicit constructor definition (or a user-defined constructor), then the compiler automatically calls the default constructor at the time of object creation. 
• The name of the constructor is always the same as the name of its class. For example, the constructor of the class named MyClass will be MyClass().
• It is important to note that the constructor might be a member function, but it does not return any value since its purpose is to provide initial values for the data members.

What Is A Destructor In C++?

The concept of destructor can be summarised as a member function whose primary purpose is to destroy the memory allocated to class objects once they have gone out of scope. That is, it is used for resource deallocation for data members that are no longer in use.

• It has the same name as the class it belongs to, but the name is preceded by the tilde symbol (~).
• The destructor does not accept arguments (i.e., there are bo input arguments), and it has no return type as it cleans up storage that is no longer accessible.
• Much like constructors, the compiler automatically calls the default destructor if the class does not contain an explicitly defined destructor.

It is safe to say that constructors and destructors are integral to writing efficient code. Now that we have a basic understanding of what the concepts of constructor and destructors entail, let's discuss the difference between constructor and destructor in C++ classes. 

Difference Between Constructor And Destructor In C++

The table below summarizes the most important differences between constructor and destructor in C++ programming. We will discuss them in detail in the sections ahead.

Constructor In C++ | A Brief Explanation

The signature for a constructor is the name of the class, followed by the parameter list, which lists the different types of parameters and the names of variables used to identify them within the class. The syntax of constructors in C++ with components is given below.

Syntax For Constructor Declaration:

class ClassName {
Access_specifier:
ClassName() {
}
};

Here,

• ClassName: Refers to the name of the class being defined.
• Access Specifier: Specifies the visibility of class members, which can be public, private, or protected. It determines whether class members are accessible from outside the class.
• Constructor: Defined within the class, initializes objects of the class. It shares the same name as the class and may include parameters for initialization. It's invoked automatically when an object of the class is created.

Need For Constructor In C++

Constructors in C++ serve several essential purposes:

1. Initialization: The primary role of constructors is to initialize objects by setting initial values to their data members. This ensures that the object is in a valid state immediately after its creation.
2. Automatic Invocation: Constructors are automatically invoked when an object is created, allowing for seamless initialization without requiring explicit calls from the user.
3. Memory Allocation: Constructors can handle memory allocation dynamically, ensuring that resources are properly allocated for the object's data members.
4. Argument Initialization: Constructors can accept arguments, allowing for flexible initialization based on user-defined values.
5. Object Initialization in Inheritance: In inheritance, constructors of base classes are automatically invoked before the derived class constructor. This facilitates proper initialization of base class members before derived class members.
6. Prevention of Uninitialized Objects: Constructors help prevent the creation of uninitialized objects, reducing the risk of undefined behavior and bugs in the program.
7. Encapsulation: Constructors are integral to the concept of encapsulation, allowing for the bundling of data and operations within a single object and ensuring that the object's state is properly initialized and managed.

Types of Constructor in C++

There are multiple constructor types in C++, each with a specific purpose. They include:

1. Default Constructor- A constructor with no parameters or with default parameters that initializes the object when no initial values are provided explicitly during object creation. This is used by default when there is no user-defined constructor.
2. Parameterized Constructor- A constructor that accepts parameters, allowing for the initialization of object data members with specific values provided during object creation.
3. Copy Constructor- A constructor that creates a new object as a copy of an existing object of the same class.

Read More: Constructor In C++ | Types, Benefits, Uses & More (With Examples)

Code Example of Constructor In C++

Code Snippet:

            #include 

class MyClass {
public:
// The class constructor
MyClass() {
std::cout << "Constructor called" << std::endl;
}
};

int main() {
// Creating an object of MyClass
MyClass obj;

return 0;
}
          

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Output: 

Constructor called

Explanation: 

In the above code, 

1. The code includes the <iostream> header file, which is necessary for input/output operations.
2. We define a class named MyClass, which contains a constructor.
3. As mentioned in the code comment, the constructor MyClass() is defined within the class. It prints the string message- Constructor called, to the console when invoked.
4. In the main() function, we create an object obj of type MyClass.
5. As soon as the object obj is created, the constructor of MyClass is automatically called and it prints the corresponding message using the cout command.
6. Finally, the main() function returns 0 to indicate successful completion.

Destructor In C++ | A Brief Explanation

Similar to constructors, the syntax of destructors in C++ also includes components that define its structure and behavior. Let's break down the syntax:

class ClassName {
public:
// Destructor
~ClassName() {
// Destructor body
// Cleanup code or other actions
}
};

Here,

• ClassName: Refers to the name of the class for which the destructor is defined.
• ~ClassName(): Represents the single destructor of the class. The tilde symbol (~) followed by the class name indicates that it is a destructor.
• Access Specifier: Specifies the visibility of the destructor. These specifiers, i.e., public, private, or protected, determine whether the destructor can be accessed from outside the class.
• Destructor Body: Contains the code block that is executed when an object of the class is destroyed. Typically, it includes cleanup actions like releasing resources, closing files, or deallocating memory.

Need For Destructor In C++

In C++, destructors are necessary for proper resource management and cleanup. Here are some reasons why destructors are needed:

1. Resource Cleanup: Destructors are essential for releasing resources acquired by an object during its lifetime, such as memory, file handles, database connections, or network sockets.
2. Prevention of Resource Leaks: Destructors ensure that resources are properly deallocated when objects go out of scope or are explicitly deleted, thus preventing memory leaks and other resource leaks.
3. Custom Cleanup Logic: Destructors allow developers to define custom cleanup logic specific to their class, such as closing files, releasing locks, or freeing dynamically allocated memory.
4. Automatic Invocation: Destructors are automatically invoked when objects are destroyed, whether due to going out of scope, being deleted explicitly, or as part of automatic memory management mechanisms like smart pointers.
5. Ensuring Object Integrity: Destructors help maintain the integrity of objects by ensuring that any cleanup operations are performed reliably, regardless of how the object is destroyed.

Types of Destructors in C++

In C++, there's essentially one type of destructor, which is the default destructor. A default destructor is automatically generated by the compiler if you don't provide one explicitly.
It has no parameters and no return type (not even void). It is responsible for releasing any resources allocated to the object, such as memory, file handles, or network connections.

Read More: Destructor In C++ | Understanding The Key To Cleanups (+ Examples)

Code Example of Destructor In C++

Code Snippet:

            #include 

class MyClass {
public:
// Destructor
~MyClass() {
std::cout << "Destructor called" << std::endl;
}
};

int main() {
// Creating an object of MyClass
MyClass obj;

// Destructor is automatically called when obj goes out of scope
std::cout << "Main function finished" << std::endl;

return 0;
}
          

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Output: 

Main function finished
Destructor called

Explanation: 

In the above code example-

1. We define a class named MyClass, which contains a destructor.
2. The destructor ~MyClass() is defined within the class. It prints the string message- 'Destructor called', to the console when invoked.
3. In the main() function, we create an object obj of type MyClass.
4. As soon as the main() function finishes execution and the obj goes out of scope.
5. At this point, the destructor of MyClass is automatically called, and the corresponding message is printed to the console, as shown above.

Difference Between Constructor And Destructor In C++ Explained

The table above gives a snapshot of the primary difference between constructor and destructor in C++. In this section, we will further elaborate on some key differences between constructors and destructors in C++:

1. Purpose:

   • Constructors are used to initialize the state of objects when they are created. They set initial/ default values to object data members and perform any necessary setup tasks.
   • Destructors, on the other hand, are used for the cleanup of resources, i.e., they perform cleanup tasks when objects are destroyed. In other words, they release resources allocated to objects and perform any necessary cleanup tasks.

2. Invocation:

   • Constructor invocation is done automatically when objects are created. They are called implicitly by the compiler during object creation.
   • Destructors are automatically invoked when objects are destroyed. They are called implicitly by the compiler when objects go out of scope or explicitly using the delete keyword for dynamically allocated objects.

3. Syntax:

   • Constructors have the same name as the class and no return type. They may take parameters for initialization.
   • Destructors have the same name as the class, but it is preceded by a tilde character(~) and no return type or parameters.

4. Overloading:

   • Constructor overloading is possible, meaning a class can have multiple constructors with different parameter lists.
   • Destructors cannot be overloaded. There can only be one destructor per class.

5. Order of Execution:

   • Constructors are executed in the order of inheritance, from base class to derived class.
   • Destructors are executed in the reverse order of construction, from derived class to base class.

Order Of Calling Constructor And Destructor In C++ Classes

In C++, the order of calling constructors and destructors in classes depends on the inheritance hierarchy and object lifetime. Here's a general rule:

1. Constructor Calls:

• Constructors are called in the order of inheritance, from the base class to the derived class.
• The Baseclass constructors are called before-derived class constructors.
• If there are multiple base classes, they are initialized in the order in which they are listed in the inheritance list of the derived class's definition.
• Once the base class constructors are called, the derived class constructor is called.

2. Destructor Calls:

• Destructors are called in the reverse order of constructors, from the derived class to the base class.
• Derived class destructors are called before base class destructors.
• If there are multiple base classes, they are destroyed in the reverse order of their initialization.
• Once the derived class destructor finishes execution, the base class destructors are called.

Conclusion

In conclusion, constructors and destructors are fundamental components of C++ classes that govern the lifecycle of objects. Constructors initialize objects, set initial values, and allocate resources, ensuring objects are in a valid state when created. Destructors, on the other hand, release resources, clean up allocated memory, and perform cleanup actions when objects are destroyed, ensuring proper resource management and preventing resource leaks.

Understanding the differences between constructors and destructors in C++ is essential for writing robust, maintainable, and overall efficient code. By mastering these concepts, programmers can create classes and objects that effectively manage resources and ensure the proper initialization and cleanup of objects throughout their lifecycle.

Also read: 51 C++ Interview Questions For Freshers & Experienced (With Answers)

Frequently Asked Questions

Q. What is the main purpose of constructors and destructors in C++?

Constructors are primarily used to initialize objects of a class, setting initial values to object data members and performing any necessary setup tasks. Destructors, on the other hand, are used to release resources and perform cleanup actions when objects are destroyed, ensuring proper resource management.

Q. How are constructors and destructors invoked in C++?

Constructors are automatically called when objects are created, either explicitly through object instantiation or implicitly by the compiler. Destructors are automatically invoked when objects go out of scope, are explicitly deleted using the delete keyword, or when the program terminates.

Q. Can constructors and destructors be overloaded in C++?

Yes, constructors can be overloaded, meaning a class can have multiple constructors with different parameter lists. This allows for creating objects with varying initial states. However, destructors cannot be overloaded; each class can have only one destructor, ensuring that resources are released in a predictable and controlled manner.

Q. Do constructors always have the same name as the class in C++?

Yes, constructors have the same name as the class and no return type, not even void. This ensures that constructors are easily recognizable and automatically invoked when objects of the class are created.

Q. What happens if a class in C++ has no constructor or destructor defined explicitly?

If a class in C++ has no constructor defined explicitly, the compiler provides a default constructor automatically. The default constructor initializes basic data members to default values (e.g., zero for numerical types, empty string for strings). Similarly, if a class has no destructor defined explicitly, the compiler provides a default destructor, which performs no additional cleanup actions. However, it's good practice to define C++ constructors and destructors explicitly, especially when resource management or initialization tasks are involved.

Test Your Skills: Quiz Time

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