Default Constructors in C++

A default constructor is a special constructor in C++ that is used for object initialization when no arguments are provided during creation. It ensures that objects are properly set up even without explicit values.

• It is automatically called when an object is created without passing parameters.
• It allows objects to be initialized with default values defined by the programmer or compiler.

Syntax

class ClassName {
public:
ClassName() {
// initialization code
}
};

Where:

• ClassName() is the default constructor.
• It has the same name as the class.
• It has no return type.
• It accepts no arguments.

Types of Default Constructor

A default constructor can be categorized into different types based on how it is defined and used in a C++ program.

Compiler-Generated Default Constructor

If a class does not define any constructor, the compiler automatically generates a default constructor.

• Created only when no constructor is declared.
• Performs default initialization of members.
• Suitable for simple classes.

class Student {
public:
    int roll;
};

int main() {
    Student s;
}

Explanation: Since no constructor is defined, the compiler automatically generates a default constructor, allowing the object s to be created.

User-Defined Default Constructor

A programmer can explicitly define a default constructor to perform custom initialization.

• Initializes data members.
• Executes custom code during object creation.
• Replaces the compiler-generated default constructor.

#include <iostream>
using namespace std;

class Student {
    int roll;

public:
    Student() {
        roll = 101;
    }

    void display() {
        cout << roll;
    }
};

int main() {
    Student s;
    s.display();
}

101

Explanation: The default constructor initializes roll with 101 whenever a Student object is created.

Initializing Objects Using Default Constructors

Objects can be initialized in different ways using a default constructor.

Member Initializer List

Instead of assigning values inside the constructor body, members can be initialized using a member initializer list.

• Initializes members before the constructor body executes.
• More efficient than assignment.
• Required for const members and references.

#include <iostream>
using namespace std;

class Student {
    int roll;

public:
    Student() : roll(101) {}

    void display() {
        cout << roll;
    }
};

int main() {
    Student s;
    s.display();
}

101

Default Constructor with Default Arguments

A constructor having all parameters with default values can also act as a default constructor.

#include <iostream>
using namespace std;

class Student {
    int roll;

public:
    Student(int r = 101) {
        roll = r;
    }

    void display() {
        cout << roll;
    }
};

int main() {
    Student s1;
    Student s2(200);

    s1.display();
    cout << endl;
    s2.display();
}

101
200

Explanation: Since the constructor parameter has a default value, it can be called both with and without arguments.

Applications of Default Constructor

Default constructors are useful whenever objects need a predefined initial state.

• Initialize objects with default values.
• Create arrays of objects.
• Required by many STL containers.
• Used in templates and generic programming.
• Simplify object creation.

Related article: Explicitly Defaulted and Deleted Functions in C++ 11