Default Constructors in C++

In C++, a constructor is a special member function that is automatically called when an object of the class is created. Among different types of constructors, a default constructor is the one that takes no arguments.

It plays a crucial role in object initialization when no specific data is provided. If you do not explicitly define any constructor in your class, the C++ compiler automatically provides a default constructor.

What is a Default Constructor?

A default constructor is a constructor that can be called with no parameters. It is either defined explicitly by the programmer or implicitly generated by the compiler.

Example 1: No User Defined Constructor:
In the above class, a default constructor will be generated by the compiler at compile time.

class GFG{
   public:
  
};

Example 2: Explicitly Defined Default Constructor:

class GfG {
    public:
    
    // Default Constructor i.e. constructor
    // with no arguments
    GfG() {}
};

In the above class, as programmer defined a constructor with no argument (default constructor), so no compiler generated default constructor will be needed.

Exanple 3:Default Constructor with Initialization:
You can also initialize class members inside the default constructor.

class GFG {
    int x;
public:
    GFG() {
        x = 10;
        cout << "x initialized to " << x << endl;
    }
};

Use Cases of Default Constructor:

• Creating arrays or vectors of objects
• Needed when using STL containers (like vector<Class>)
• Used when objects need to be initialized to default values
• Required in generic programming and templates
  

#include <iostream>
using namespace std;

class GFG {
    string name;
    int roll;
public:
    // Default constructor
    GFG() {
        name = "Unknown";
        roll = -1;
        cout << "Default constructor called\n";
    }


    void display() {
        cout << "Name: " << name << ", Roll: " << roll << endl;
    }
};

int main() {
    GFG s1;               // Calls default constructor
    s1.display();
    
    return 0;
}

Default constructor called
Name: Unknown, Roll: -1