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.

• A default constructor is the one that takes no arguments. It is either defined explicitly by the programmer or implicitly generated by the compiler.
• If you do not explicitly define any constructor in your class, the C++ compiler automatically provides a default constructor.

Example 1: No User Defined Constructor

In the below class, a default constructor will be generated by the compiler at compile time.

class GFG {
public:
};

Example 2: Explicitly Defined Default Constructor

In this example, a constructor with no parameters is explicitly defined. Since a constructor already exists, the compiler will not generate one.

class GFG {
public:
    GFG() {
        // default constructor
    }
};

Objects of this class can be created without arguments.

Example 3: Default Constructor with Initialization

You can also initialize class members inside the default constructor.

#include <iostream>
using namespace std;

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

Explanation:

• The default constructor initializes the data member x.
• It runs automatically at object creation time.
• Useful for setting initial default values.

Complete Example

#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

Explanation

• s1 is created without arguments.
• The default constructor initializes name and roll.
• display() prints the initialized values.

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