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Constructor:

• A constructor is a special member function of a class and shares the same name as of class, which means the constructor and class have the same name.
• The compiler calls the constructor whenever a class object is created; it allocates memory to the object and initializes class data members with default values or values passed by the user when creating an object.
• Constructors have no return type because their only task is to create and initialize an object.

Syntax of defining a constructor:

class class_name  
{  
//Statements 
public  
class_name ([parameter list])  
{  
  //Constructor's Body 
}  
};  

Types of Constructors :

There are four types of constructors in c++

• Default constructor
• Parameterized constructor
• Copy Constructor
• Dynamic Constructor

Default constructor:

• Default constructor is also known as a zero-argument constructor, as it doesn’t take any parameter.
• If the user does not define it, the compiler creates it on his own.
• The default constructor always initializes data members of the class with the same value they were defined.

Example:

#include <iostream>
using namespace std;

class Student{
  
  // declaring private class data members 
private:
  string name;
  int age;
  
public: 

  // declaring constructor
  Student()
  {
      cout<<"Default constructor is called"<<endl;
      name = "student";
      age = 12;
  }
  
  // display function to print the class data members value 
  void display()
  {
      cout<<"Name of current object: "<<name<<endl;
      cout<<"Age of current object: "<<age<<endl;
  }
  
};
int main() 
{
    // creating object of class using default constructor
    Student obj;
    
    // printing class data members 
    obj.display();
    
    return 0;
}

Output:

Default constructor is called
Name of current object: student
Age of current object: 12

Parameterized constructor:

• Parameterized constructor is used to initialize data members with user-supplied values.
• This constructor is essentially a more advanced version of the default constructor.
• We can define multiple parameterized constructors based on the needs of the user, but we must adhere to the rules of function overloading.

Example:

#include <iostream>
using namespace std;

class Student{
  
  // declaring private class data members 
private:
  string name;
  int age;
  
public: 

  // declaring constructor
  Student(string n, int a)
  {
      cout<<"Default constructor is called"<<endl;
      name = n;
      age = a;
  }
  
  // display function to print the class data members value 
  void display()
  {
      cout<<"Name of current object: "<<name<<endl;
      cout<<"Age of current object: "<<age<<endl;
  }
  
};
int main() 
{
    // creating object of class using default constructor
    Student obj("Rabecca",15);
    
    // printing class data members 
    obj.display();
    
    return 0;
}

Output:

Default constructor is called
Name of current object: Rabecca
Age of current object: 15

Copy Constructor:

• A copy constructor is used when we have an object of a class and want to create a copy of it in a new declared object of the same class.
• The compiler provides a default copy constructor for each class, and users can define their own.
• It only accepts one argument, which is an object of the same class.

Example:

#include <iostream>
using namespace std;

class Student{
  
  // declaring private class data members 
private:
  string name;
  int age;
  
public: 

  // declaring constructor
  Student(string n, int a)
  {
      cout<<"Default constructor is called"<<endl;
      name = n;
      age = a;
  }
  Student(const Student& obj)
  {
      cout<<"Copy constructor is called"<<endl;
      name = obj.name;
      age = obj.age;
  }
  // display function to print the class data members value 
  void display()
  {
      cout<<"Name of current object: "<<name<<endl;
      cout<<"Age of current object: "<<age<<endl;
  }
  
};
int main() 
{
    // creating object of class using default constructor
    Student obj("Rabecca",15);
    
    // printing class data members 
    obj.display();
    Student obj2(obj);
    
    // printing class data members 
    obj.display();
    
    return 0;
}

Output:

Default constructor is called
Name of current object: Rabecca
Age of current object: 15

Copy constructor is called
Name of current object: Rabecca
Age of current object: 15

Dynamic Constructor:

• The constructor is known as a dynamic constructor when memory is allocated dynamically to data members at runtime using a new operator.
• The only difference between this constructor and the default or parameterized constructor is that it uses a new operator to allocate memory.

Example:

#include <iostream>
using namespace std;

class Student{
  
  // declaring private class data members 
private:
  int* age;
  
public: 
  Student(int* Student_age)
  {
      cout<<"Constructor for age is called"<<endl;
      
      // allocating memory 
      age = new int;
      age = Student_age;
  }
  
  // display function to print the class data members value 
  void display()
  {
      cout<<"Age of current object: "<<*age<<endl;
      cout<<endl;
  }
  
};
int main() 
{
    // creating objects of class using parameterized constructor
    int age = 25;
    Student obj1(&age);
    
    // printing class data members for first object 
    obj1.display();
    
    return 0;
}

Output:

Constructor for age is called
Age of current object: 25

Destructors:

• The destructor function is the inverse of the constructor.
• The compiler calls a destructor when an object is destroyed, and its main function is to deallocate the object’s memory.
• When the program terminates, the object is destroyed, or local objects of the function exit the scope when the function terminates, or in any other case.
• Destructors have the same properties as constructors and cannot be overloaded like constructors.
• Destructors accept no arguments and have no return type or value.

Syntax for Destructor:

class class_name  
{  
//Statements 
public  
~class_name ([parameter list])  
{  
  //Destructor's Body 
}  
};  

Example:

#include <iostream>
using namespace std;

class Student{
  
  // declaring private class data members 
private:
  string name;
  int age;
  
public: 

  // declaring constructor
  Student()
  {
      cout<<"Default constructor is called"<<endl;
      name = "student";
      age = 12;
  }
  ~Student()
  {
      cout<<"Destructor is called"<<endl;
      name = "student";
      age = 12;
  }
  
  // display function to print the class data members value 
  void display()
  {
      cout<<"Name of current object: "<<name<<endl;
      cout<<"Age of current object: "<<age<<endl;
  }
  
};
int main() 
{
    // creating object of class using default constructor
    Student obj;
    
    // printing class data members 
    obj.display();
    
    return 0;
}

Output:

Default constructor is called
Name of current object: student
Age of current object: 12
Destructor is called

Note: also read about Function Overloading in C++

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